Those thumbs are to help you build a queue. They let you approve/disapprove of the picks, build a queue that you are 100% confident in, then walk away and take a shower and not worry about being subject to a song that you absolutely hate while you are trapped and soaking wet.
There is a detailed discussion of the thumbs and what they mean/how to think about them here:
We will probably move away from thumbs iconography on the queue screen in the future. It has actually been in the product with those icons from the very start–and it’s surprising us how many people are experiencing this as a totally new source of confusion this week.