Mass is what agar cells are made of and what they need to eat to get bigger.
The default starting mass without an account is 10.
Earning Higher Starting MassEdit
Main page: Levels
Players may register an account in agar.io with either Facebook  or Google+[], which will allow them to start with a higher mass. If a player levels up with that account, they will receive +1 starting mass. This rule is broken when the player reaches level 24 when logged into that account, and the maximum starting mass is 43 with the exception of a starting mass boost multiplier. See for more information about greater starting mass at the Levels page.
Main page: Virus
In FFA mode, if a player runs into a virus that player will gain the amount of mass the virus has, although still splitting the player's cell as a virus normally does. The default virus mass is 100, unless a player throws mass into it. When a player has 16 cells, a player may gain that virus's mass without splitting any more if the virus is eaten.
Other Smaller CellsEdit
Main page: Ejecting
You can eject your mass out of yourself to give it to other get you by pressing W
Other players instead of ejecting mass, will split and give at least one of their cells, feeding their friends.