What is the molar mass of sodium selenide?

Well Sodium Selenide has a formula of Na2Se. Let's take a look at their molecular masses

Molecular Mass of Sodium = 22.99

=> 2 Sodiums = 45.98g

Molecular Mass of Selenide = 78.96

=> The Total Molecular Mass is 45.98 + 78.96 = 124.94 g mol-1

What does that figure mean?

This is the amount of Sodium Selenide that is required to make a 1 mole solution of this substance in a litre of water.