The preposition across shows movement or position from one side to the other.
- The child ran across the road. (From one side of the road to the other)
- There is a bridge across the river.
- Can you jump across the stream?
Across can also mean on/to the other side.
The library is just across the road.
- I live across the street.
Across and Through
The difference between across and through is similar to the difference between on and in. Across is used for a movement in a two-dimensional space. Through is used for a movement in a three-dimensional space.
- We walked across the field. (We were on the field.)
- He drove through the tunnel. (He was in the tunnel.)
- We slowly walked through the wood. (We were in the wood.)