Antiphase generally refers to phase coherence between the left and right channels. If they're fully in-phase then you essentially have a mono signal, if they're full out of phase (or 180 degrees out of phase) that is the same thing as anti-phase, and the two signals would cancel if summed to mono.
Stereo works by having phase differences between the two channels, mainly because the contain DIFFERENT instruments or elements so they cannot be be perfectly in phase. These are typically small differences though, 0-45 degrees of phase shift, maybe up to 90 degrees, that's what gives your width and stereo image, but when things get closer to full antiphase it A) becomes uncomfortable to listen to, and B) largely disappears if listened to on a mono system.
Best way to keep it under control while still maintaining a nice stereo image is to keep your low end mostly mono, as well as your big transients like kicks and snares, and pan different elements of to the sides instead of artificially 'widening' one element with delays/doublers.
