First, you need to know the origins of the phrase. It is the opposite of "politically incorrect". 20 years ago, we didn't tell people "be politically correct" we told them "don't be politically incorrect" or "you can't say that, it's politically incorrect." But that phrase itself went through an interesting transformation. It was first said as a mocking thing by people who liked to say racist or sexist things and complained that now they couldn't, because it was "politically incorrect." The connotation was that it wasn't incorrect at all, and was fine, but some people (you know, those people who bring politics into everything) were stupidly complaining and saying it was wrong when everyone knows it's fine. Then, over time, the people being complained about embraced it, used the phrasing themselves unironically, and came to use the opposite phrase, being politically correct, as something to aim for.
A quick first approximation is that being politically correct is being polite. You don't ask intrusive questions. You don't tell people in group X your thoughts on why X people are not good at a particular thing, why most X people are poor, how much better it is to be in your group than group X. You don't have to do the opposite and say X people are great and better than you, you don't even have to stop thinking that X people are disadvantaged, you just need to refrain from sharing that opinion with people in group X or in a context where they (or people who care about members of group X) might hear you and be hurt. For example, if you're with a number of officemates, you don't start to explain why you think same sex marriage should not be allowed, or women shouldn't be able to vote, that sort of thing.
A more advanced version, for those who have practice with the "just don't be rude; keep quiet if your thoughts and opinions would hurt those who hear them" version is to expand your own awareness and consider the possibility that people in those "lesser" groups have something valuable to contribute: in the workplace and in general. That in some cases they bring an advantage; different backgrounds and experience, different priorities, that can strengthen your company, make the product or service you're selling more useful, or get the job done more quickly or smoothly. Be willing to look past your first impressions that someone is black, a woman, gay, way more religious than you or whatever and see a person who has true strengths that are welcomed and needed. Most people find this hard to do until they've spent pleasant time with people in those groups - and you get that from the "be polite" version of being politically correct. Once you start to think this way, you find yourself listening to suggestions from people you once ignored, defending them when other people make hurtful jokes or comments, and noticing when your workplace acts to exclude people who want to be included.
Most of all, don't be politically incorrect in your humour, your political opining, your off-the-cuff comments. Don't assume the woman in your team wouldn't like some activity you're inviting all the men to, or doesn't drink beer. Don't assume the unmarried man on your team isn't in a committed long term relationship, and make decisions based on his having free time. Don't assume you can tell what sport a person likes to watch based on their skin colour. If you need a single rule it would be this: the rest of the world usually doesn't need your opinion. If you can't share it without hurting someone (or you're not sure because last time you shared a similar thing someone got all offended even though there was absolutely no reason to because it's science and science is completely unbiased and rational), just save it. Share it another time with people you don't have to work with, attend university with, and so on.
