What is bounce rate?

That depends on your site and what your visitors are intending to do there.

Bounce rate is merely an indication that visitors have arrived at your site on one page, and left again without visiting any other pages. It is not in itself an indication that there is a problem. If you have a single page site, you will have a bounce rate of 100%, simply because there are no other pages for your visitors to go to.

As I said recently in another topic:

So you need to look at bounce rate along with other indicators to decide whether or not you have a problem which needs addressing.