Piggybacking is the technique of attaching ("riding along with") an acknowledgement to an outgoing data frame, instead of sending the ACK as a separate, standalone frame.

In a full-duplex or bidirectional connection, both sides are typically sending data to each other. Rather than wasting a whole frame just to carry an ACK, the ACK field is embedded within the header of the next data frame that station happens to be sending anyway, so a single frame carries both new data and the acknowledgement for previously received data.

Benefit: it reduces the number of frames transmitted (and hence the overhead) needed to keep both directions of communication flowing reliably.