A perch or tactical is a bookmark located at least 150km away from a fixed object, typically a stargate or station. You should make perches in systems you frequent. Warping to a perch puts you on grid with your target so you can observe the target at a reasonably safe distance that also allows direct warping.
Be careful with inline and straight up (or down) perches. Being more common, these are less safe. But you can use them to warp in on enemies.
150km is minimum warpable distance
Perches farther than 150km are safer, and it's good to have variety
Several perches (at least 150km apart) can help you avoid enemies trying to burn towards you.