Between is a preposition
I’m not sure where Castelldefels is, but I know it is somewhere between Barcelona and Sitges.
“Between” can be used to express where something is when we are not sure of its exact location.
We also use the preposition “between” when we don’t know the exact amount of something.
e.g. The tickets for the show will cost between $10 and $20, I’m not sure.
e.g. Barcelona is located between the mountains and the ocean. It’s very beautiful!
e.g. My and is stuck between the wall and the rock!