Abstraction is removing unneccesary details from a problem. Uses of abstraction include:
Simplifying something by removing anything irrelevant to the problem, e.g. the London Underground map doesn't give the actual locations of stations and train lines in London; only how they're connected together.
Grouping different problems together with similar characteristics, and simplifying the logic of them to be solved using similar algorithms, e.g. a pathfinding algorithm can be used to find a driving route in Google Maps, or a network path for packet switching.
Programmers don't need to know how e.g. integers or strings are stored in memory when programming in high-level languages, which saves them time.