What does abstract mean in science?

An abstract idea is one that is just an idea, and can't be expressed in concrete form.

An abstract in a paper or thesis is a brief description of what you will find in the paper. It usually summarizes the experiments done, and gives a teaser of the results. They are usually written in such a way that people who are interested in the topic will have to buy the paper or book if they really want to understand the topic.