The pieces have point-values which reflect their degree of mobility.
However, the position on the board can affect the mobility and roles of the pieces – therefore the points are only an estimation of their value in average situations.
* In most positions a bishop is actually worth a bit more than a knight
** The king is a powerful piece in the endgame-stage of the game
An advantage refers to any factors in the position that may improve your chances of winning the game.
A material advantage goes to the player who has more and/or stronger pieces. The simplest way to determine which player has a material advantage is to compare the point-values of the remaining pieces on the board.
Material superiority isn’t the only factor that determines who has an advantage, but it is often the most important one.