Salam alikum means upon you peace. This is what the angels will say to the people when they first enter heaven. The greeting is peace/ salam in Arabic.
The full greeting is Salam alikum rahmutallah wa barakutu/May the peace, blessings and mercy of God be upon you; saying this gets you thirty rewards.
Salam alikum rahmutallah gets you twenty and salam alikum gets you ten.
This comes from the time in heaven when God/Allah commanded Adam upon him peace to greet the angels. He said, "Salam alikum rahmutallah" the angels replied, "Salam alikum rahmutallah wa baraktu." Adding blessings.
Amazing that just by greeting someone you get rewarded for saying a few words!