Interaction contrasts

Can anyone explain to be in english what they are? I have a 4x3 mixed factorial and I know how to compute the interaction contrasts, but once I come up the p values, I don't know what it means. If an interaction is significant... ?!?! what? does that mean that as one increases the other decreases, or they increase/decrease together??
if an interaction is significant it means there is the lack of one variable to have the same effect across the levels of the other variable.