From Quake Wiki
This is the brush entity that allows the player to move from one map to the next. The sole keyvalue for this entity, "map", is simply the filename of the map to be started after the trigger_changelevel is touched. Don't use any .BSP extension in this value. If your map is called nextmap.bsp, then just use nextmap in the value. There is also a flag you can use to specify whether or not you want to use a level intermission (info_intermission) camera when the "monsters killed/secrets found" screen comes up.