In recent years, politically correct language has gained a somewhat bad reputation -- denigrated to a manipulative tool used to appease overly-sensitive citizens. Although I agree that sometimes politically-correct language can be taken too far, I am a firm believer in its value. The words you use shape your thoughts. If you are accustomed to speaking about certain people in a dehumanizing way, your thoughts will be shaped into thinking that they are less than human.
People-first language is about speaking of people in a way that honors their dignity, humanity, and individualism. It seeks to reduce misunderstandings and prejudice.
Here are some examples of people-first language and non-people-first language, respectively:
He has a learning disability vs He is learning disabled
She has a physical disability vs She is crippled
John uses a wheelchair vs John is wheelchair-bound
Sue has a mental illness vs Sue is crazy
Can you see the differences between the two? People-first language honors the person first and mentions the disability or challenge as a secondary feature. Don't get carried away -- sometimes people-first language can get repetitive and awkward if you take it too far. The important thing to remember is to be mindful of being respectful.