Critical thinking as a term is all about applying reasoning and judgement to your arguments. It's about making sure you've asked yourself all the the possible awkward questions, so that you can be sure your reasoning is watertight, and that you have not allowed any personal bias or mistaken thinking to creep in. It can help you decide what is relevant to your study, and what can safely be left out, and help you decide whether your ideas are valid. It's important to collect enough suitable material to provide good evidence to allow you to make a reliable evalution.
At the simplest level it's a matter of using your common sense and being objective, but it might be something you want to pursue further, depending on your subject area. If your arguments are complex or radical, you might want to learn more about logic to make sure you are not going astray. There are various tools you can use for reasoning and presenting your arguments, e.g. inductive or deductive reasoning.
And of course it's about developing good habits as well - patience and thoroughness are essential, but you also need to cultivate a critical, inquiring mind and a sense for new avenues that might be worth exploring.
Here are some views on what Critical thinking is:
Sometimes when you look at all the things that researchers should be doing it seems as if something important at the heart of it all might be missing. There are so many activities to be performed, and yet surely a lot of the time researchers ought to spend time just -
Cognitive abilities – analysing / synthesising / critical thinking / evaluating / problem solving
Creativity - Inquiring mind / Intellectual insight / Innovation / Argument construction / Intellectual risk