"You are welcome."
Sama-sama "same same" so its like if someone said thank you, and you said "you'd do the same for me". It's casual way of saying you're welcome.
Selamat is a strange one for english, but is closest to congratulations. Datang means come.
So congratulations your coming here. Is somewhat what selamat datang means. In the same way selamat pagi is Congratulation the morning.
Hope that helps somewhat.