Salmon live their first years in the river. Then they develop to tolerate salt water. This is a process we call smoltification. The salmon is now a smolt.
The smolt swim through the fjord and out into the sea. Here they spend one to four years hunting for food and growing up.
When the salmon are big enough, they swim back home to reproduce. We call this spawning. The salmon almost always find their way back to the river they grew up in.