Temp-on-rez is a prim setting which will keep a prim ‘alive’ for a fixed amount of time, after which it will be removed. It is a very handy feature with a number of legitimate uses, the most common of which I suppose are bullets and vendor displays (which temporarily rez examples of the items for sale).
Unfortunately it is sometimes used as a way of circumventing Second Life’s prim limits. There are gadgets you can get which will essentially keep temp-on-rez items permanently alive (basically they re-rez the items every time the temp-on-rez reaches its time-out).
If you’ve been tempted by these gadgets, or if you’re actually thinking of trying to create one, please, please don’t.
The prim limits are there for a reason. The more prims there are in a sim, the heavier the load on the Second Life servers. Using the temp-on-rez to bypass the limits simply increases the load, and makes the Second Life experience even worse than it already is, not only for other users but for you (the temp-on-rez abuser!) yourself.
Second Life is struggling enough as it is. Don’t make it worse.