This is the 727th most frequent Swedish word.
längs
"Along"
In this sentence, "längs" is used to indicate motion or presence alongside a physical feature, the beach.
Vi promenerade längs stranden och njöt av solnedgången.
We walked along the beach and enjoyed the sunset.
Here, "längs" describes a path taken parallel to a natural feature, the river.
Fåglarna flög längs floden och följde dess krökningar.
The birds flew along the river and followed its bends.
In this instance, "längs" specifies a location parallel to a path or road where something is observed.