Using almost any ability in GURPS requires a 3d6 roll, and this bonus makes it easier.
For example, assume you bought the Warp ability (p B.97-98) as psionic teleportation. To use it, you need to roll under your IQ on 3d6. If you have Teleportation Talent, you need to roll under IQ+Talent.
With no Talent and IQ 12, you could teleport up to 10 yards by rolling 12 or less. However, if you wanted to teleport 500 yards, that's -3 to your roll, so you'd need to roll 9 or less.
If you had three levels of Teleportation Talent, giving you +3, that makes your basic teleport roll 15 or less, and your 500 yards roll is 12 or less.
For people used to rolling d20s, this may seem like a lot of fussing for a fairly small difference in odds. It isn't. With a 3d6 roll, a 15 or less roll is just over 95% chance of success, a 12 or less is 74% and a 9 or less 37.5%. Betting on making low rolls in GURPS, Hero System, or any other game that uses this kind of dice mechanic, is quite unwise.
There is an add-on book for GURPS, Psionic Powers, which gives you a much more fully developed psionics system, as an upgrade to the Basic Set. That gives you more flexibility and a wider range of capabilities.