The meaning of "people that matter".

This phrase would depend on the subject, and the speaker as well. But I think you got the right idea. In other words, People that matter to me would be different than people than matter to you. Especially since I do not know any of your relatives or friends. And you and I are not familiar with any of the same people. Here it is used in a sentence. The people that matter the most to me are my boyfriend, my mom and my brother. I hope that this is somewhat helpful to you!