The Midheaven is the sign that was at the point in the sky where the ecliptic intersects the meridian, when you were born. What this means is basically the highest point the Sun would reach at midday.
To calculate it, you need your birth time and your location on Earth. Each sign moves through this point for only about two hours, so having an accurate birth time is necessary. In quadrant house systems, the Midheaven forms the cusp of the tenth house. In Whole Sign Houses, it is usually in the 9th or 10th house, although it can be anywhere from the 8th to the 12th in extreme latitudes.
The Midheaven is the interface between you and the public. (How people see you when they get to know you personally is your Ascendant.)
While you have consciously worked on your career and public image, you may not be aware of the full extent of the "why" behind all your actions. For example, if your MC is in Libra, you may not realize how much fairness and a sense of abiding by principles has been guiding you in your work. Libra is ruled by Venus, and if your Venus is in Leo, you might not realize how much being acknowledged and recognized matters to you, and how you like to add a bit of flair to how you present yourself in business settings.
To get a full picture of your MC, you can look at:
See the Ascendant article for the rulers of the signs.
These planets tend to be the most challenging to have on the MC.
Saturn conjunct the Midheaven. This definitely has positives, like responsibility, discipline, practicality, and the willingness to work hard toward long-term goals. The downsides are that wherever Saturn is can be a place of limitation and constriction. This can show up as obstacles you need to overcome to grow your career, or a delay in starting your career. You may struggle with self-doubt or fear of failure or making mistakes. Saturn lessons are always about doing the hard work anyway, through which we develop maturity, wisdom, and mastery.
Uranus conjunct the Midheaven. "Unconventional" is the word most applicable to Uranus in a career context. You don't want to do things the way they've always been done, and you have lots of good ideas for how to change things, in a way that will benefit everyone. You deeply care about humanity and doing things the better way. Not everyone will understand you, but you usually don't mind. The main downside of Uranus is that it can be a chaotic energy, and end up disrupting things that aren’t all that helpful to disrupt. It can also come across as aloof or detached and unemotional, and you may need to try extra hard to bridge this gap so people understand and can align with your plan on an emotional level. Humans are pretty universally afraid of change and comfortable with the familiar—it's just how we are wired. Try to understand that, even if it doesn't come naturally to you (or find a partner who does), and you'll get more buy-in for your vision.
Neptune conjunct the Midheaven. You may be drawn to spiritual or creative fields, and a sense of compassion and maybe inspiration will be part of your public face. The downside of Neptune is that it blurs boundaries and causes confusion. You could feel vague or passive about your career path and it could be challenging to clearly define what you do or what direction to take. Neptune also creates sensitivity, so a public role that exposes you to harsh criticism could be very difficult to manage. There may be a tendency to want to escape or avoid harshness. To combat this, get very clear on what you can and can’t handle and let go of what isn’t a good fit. Neptune is also very into self-sacrifice and over-giving out of a sense of guilt or need for redemption. Look at these kinds of patterns honestly and let go of any pattern of martyrdom; you deserve a career you enjoy and that doesn't harm you.
Pluto conjunct the Midheaven. You are seen as intense, powerful, magnetic, and possibly intimidating. Power struggles are the main problem with Pluto. Whether you like it or not, your energy lights up power dynamics when you walk into a room, and things can get messy. You may not mind the intensity, but other people probably do. You may benefit from learning very clear communication skills and being transparent even when you'd rather not. The upside is that you have a spidey-sense for anything underhanded going on, and provided it's not miscalibrated, it can be very handy and self-protective. You can see under the surface of things and your insight can help you navigate complex situations. You may have a tendency to become obsessive; keep in mind that sometimes it's better to just let something go.