What does it mean when you have blue dots on the bottom of your feet?

It means at least one of two things, and possibly both:

-- You have been walking around barefoot on magic markers.

-- You need to see a podiatrist (foot doctor), an expert, who will examine
the area, and then tell you ... based on his expert examination ... either
that it's nothing, or else that it's something that should be treated because
it doesn't belong there.

DO NOT shop on the internet for health or medical advice.
It's possibly the stupidest thing a person can do.

To kill bacteria from our body especially from stomach