A share is defined by the amount of seats you have in an IMO (Initial Masternode Offering). Every IMO has a predefined amount of seats available. If an IMO has 100 seats and the you have bought 10 seats, your total share in this IMO will be 10%.
Owning 10 seats in a 100 seats IMO will also gives you 10 votes when community decisions have to be made. 1 seat also means 1% of the monthly reward. 10 seats means 10% monthly rewards.