Evaluation functions calculate approximately the score which can be prove of guaranteed if a particular world state is reached. Same like in chess, evaluation functions have been calling such as long before computers came along. Individual that function probably simple counts the number of pieces on the board for a particular player. Although there is A more convoluted function scores much more for the more influential pieces like rooks and queens: each one of them is all the time pawn is worth 1, knights and bishops score 3, rooks score 5 and queens score 9. Every single one these scores are needed in a weighted linear function, well just all doing the products are added up to where the number of pieces of a definite type is multiplied by a weight. Used for instance, player one has 6 pawns, 1 bishop, 1 knight, 2 rooks and 1 queen in the same, so after this in a particular board state, then the evaluation function, f for that board state, B, would be calculated as follows:
f(B) = 1*6 + 3*1 + 3*1 + 5*2 + 9*1 = 31
The numbers in bold are the weights in that kind of evaluation function that is the scores assigned to the pieces.