in

Used to indicate location or position within a larger area, enclosure, or container; also used to express presence, involvement, or a state or condition.

• The books are in the library. — The books are located inside the library.
• She works in the medical field. — Her profession is within the medical sector.
• They are in agreement on the proposal. — They share the same opinion about the proposal.

Indicating movement towards or presence at the inside of a place or thing.

• Please come in. — Please enter the building or room.
• He's not out, he's in. — He is currently inside and not outside.
• The train pulled in to the station. — The train arrived at the station and moved to its designated position within it.