that which trickles; also, in the world of Windows programs, a particular program (generally called fsg.exe or fsgsomething.exe). When installed (normally without your knowledge), Trickler in turn installs a registry key that causes it to be run at startup time.
Trickler's purpose is to slowly download some other program and subsequently install it. This program is typically the Gator adware/spyware system. Reasonable speculation says that this sneaky and slow installation is done so that the user does not associate the Gator program with the program trickler came with.
One program that installs trickler is Audiogalaxy, but there may be others.
In my experience, the tricler executable can be deleted without any consequence, and you should probably do so.