In Path of Exile, every area is instance based. Players may know, when you enter an area, a new instance is created. It remains open as long as you are in it, in addition, it remains open for more or less 15 minutes after you have left it.
You can force – create as many instances of the area as you want – it is useful if you want to repeatedly run the same zone for leveling or divination card drop purposes because creating new instance means having it populated with monsters again.
To create a new instance, hover over the entrance, hold CTRL key and left click on it. Doing so will bring up instance creation menu where you should choose “NEW” option. Below I have attached an image of the menu. Just a note: you can also create a new instance of the zone by CTRL – clicking on the waypoint which is associated with a given zone.
Something to add if we are talking about instances is that for some areas instances cannot be generated repeatedly. Some examples:
1) Endgame maps (one instance with 6 portals to it is created).
2) Vaal Side areas (one instance, once you clear it from monsters, you cannot generate new one).
3) Boss rooms for Shaper, Atziri, Pale Court, etc.
Hopefully the above guide will be helpful to you.