Critical thinking is the ability to evaluate information instead of accepting it.
For example, when you read a newspaper, a critical thinker examines:
-Who's writing the newspaper?
-What are their biases and objectives?
-What sources do they use for their information?
-Can I trust them to report honestly?
-Who pays the writers? How does the affect their honesty?
-Is this article logical? Are there any logical flaws in it?
-How does this news article compare with other articles by different newspapers?
Everyone does this to some extent. Critical thinking is just having a more organized habit of doing it.
It means to think well using reasons and cheking every detail side by side to give a good answer or explaination.
It means that you think through something to make sure that it is logical and to find any inconsistencies in it before believing it.