Benchmarks run a series of tasks of different types to give a figure that can be compared across systems.
For example, your phone will do certain calculations, converting a 1080p video from one format to another, generating however many polygons at a certain resolution.
How well these figures translate to real-world speed is often a topic for debate, especially when comparing phones that run on a different operating system, but it gives you SOMETHING to compare.
Download Geekbench I guess.