the table itself is made of characters which show how a particular irreducible representation changes when a particular symmetry operation is applied (put to it). any symmetry operation in a molecule's point group acting on the molecule itself will leave it unchanged. but for acting on a general entity (thing), such as a vector or an orbital, this does not have to be what happens. the vector could change sign or direction, and the orbital could change type. for simple point groups, the values are either 1 or -1: 1 means that the sign or phase (of the vector or orbital) is unchanged by the symmetry operation ("symmetric") and -1 denotes a sign change ("asymmetric").