Technically speaking a non-linear structure is defined as anything that is not taken in natural order, going from first to last. More loosely defined as a theatrical convention, it means that the playwright can use a self-created structure using flashbacks or snapshots of a place in time without a logical sequence.
If this convention is used in a live performance without the technical 'cues' available in film, the production team has to take care not to create an unpredictable and unstable environment in which the audience is lost in chaos.
"Always smile at your enemies. It will keep them on their toes"