A biscetor is a line that curs another one exactrly at its midpoint. If the diagonals of the paallelogram bisect each other, this means that both cut the other in exact halves.
For example
For the square ABCD, its diagonals BD and AC cut each other in point M.
M is their midpoint, which means that the haves of each diagonal are the same:
For diagonal AC you get that AM=MC
For diagonal BD you get that BM=MD
This means that the diagonals bisect each other and have the same midpoint M