It's like an abacus, the old calculator which uses beads on a wire - the beads can slide along the wire to either side, but they stay on the wire. They can slide in either direction until they're all on one side. Once the beads are all on one side it means the channel can only send (no inbound liquidity beads left to slide) or only receive (no outbound liquidity beads left to slide.)

Even though some of a nodes channels have all the liquidity on one side, it can still function, but now it's like navigating down one-way streets where the options are more limited and it might take more turns to reach the destination.