I need some advice, I had some crypto a few years ago and then sold it and made some good profits and I am now planning to buy some crypto once again and I admit that I am outdated on the crypto world right now. Can someone give a brief explanation of what DeFi is and what are your top 5 coins/tokens and where do you usually trade it. Thanks! 
Do your own research, here I can't recommend a suitable coin for you to invest in, because my taste and yours is not necessarily the same. But you can see this article TOP CRYPTOCURRENCY PROJECTS TO WATCH OUT FOR IN 2021 to be used as an investment reference that is right for you. It's hard to explain it in simple terms but you can read an article[1] which is something like this.
[1] What Is DeFi?
Regarding your question What Is DeFi? You can find the answer here.[1] What Is DeFi?