The phrasal verb "get across" means to successfully communicate. If you are able to "get across" an idea, it means that the people who are listening or reading are able to understand what you wanted to communicate:

The main thing I want to get across to you today is the importance of slow, steady effort toward your goals.

People use "get across" for complicated ideas, such as logical arguments in essays. You wouldn't use "get across" to talk about communicating the price of something, for example. That's too simple of an idea.