Refer to figure.
Often aircraft will have a CG that changes significantly during flight, mostly due to fuel burn. This has an affect on the aircraft's flight characteristics, being that a forward CG requires more nose-up trim, which causes a downforce on the horizontal stabiliser, and therefore means that the main wing is lifting a heavier load. This leads to increased drag (sometimes called trim drag), increased stall speed, lower maximum altitude, etc.

This is oposed to an aft CG, which reduces all those negative effects, for the trade-off of reduced stability.

This makes it useful for us to be able to change the CG of the aircraft so we do not have to contend with as much trim drag. To do this, a trim fuel tank can be fitted in the rear of the aircraft, which is usually tranferred and then burnt at the correct time to ensure peak efficiency, with the best possible CG position. This is the reason they are fitted in the tail of the aircraft, as that is the place furthest away from the CG, to have the greatest effect.

An interesting side note, Concorde had such trim tanks in the tail of the aircraft, and fuel would be pumped to them to bring the CG backwards when accelerating through the transonic range, in order to counteract the effects of Mach tuck. This drastically reduced the drag of a conventional Mach trim system, and the reverse would occur when the aircraft decelerated back to subsonic flight again.