The phrase "pinned and helpless" is composed of two parts in the actual text:
If you win, your foe is pinned and helpless.
The italics in the original text indicate that pinned is a GURPS technical term, while "helpless" is merely explanatory prose for the benefit of the reader.
The definition of pinned appears in the boxed rules† Actions After Being Grappled on the facing page (371):
If you have been grappled, you cannot take a Move maneuver unless [you're much stronger]. [A list of specific maneuvers] – and ranged attacks – are completely impossible. If you are pinned, you can't take any maneuver that requires physical movement!
In context, the meaning is that the restrictions from being pinned are in addition to the restrictions from being grappled, so being pinned means that not only can't you move in the sense of locomoting, but you can't move any part of your body at all.
As explanatory text goes, "helpless" is a pretty good plain-English (non-game-term) description of the game-term effects of being pinned. At this point, the game hands over to the GM the responsibility to interpret and adjudicate (and apply other rules as necessary) the in-context ramifications of being helpless and unable to move.
† GURPS 4e has a habit of doing this: presenting something as a technical term without an obvious definition, and burying its definition in what feels like a throw-away sentence in some boxed text one or even several pages later. Always hunt for boxed text when the rules act like you should know what a term means but it's not immediately obvious.