Tripping the boundary layer refers to the action of artificially transitioning a laminar boundary layer into a turbulent one. It can happen intentionally (via turbulator) or unintentionally (via imperfect aero-smoothness, such as rivets, bolts, counter-sinks). At the point of boundary layer tripping, the surface protrusion destabilizes the laminar boundary layer and causes the transition to occur earlier than via natural Tollmien-Schlichting instability.
The following is an example of a turbulator tape. It is attached spanwise along the surface of the wing (facing flat against the surface) at the desired chord-wise location where the laminar-turbulent transition is to take place:
Since turbulent boundary layer has higher skin-friction drag than a laminar one of the same Reynolds number, unintentional boundary layer tripping is undesired. That's why wing surfaces must be made flush and smooth. At the same time, since a turbulent boundary layer is much better at resisting flow separation than a laminar boundary layer, tripping the boundary layer can be used to improve stability & control at high angle of attack at the expense of increased skin-friction drag.