They grant the users abilities and attributes identical to that of a super transformation, such as added power, the ability to fly and invincibility. It requires Ring energy to be maintained (the player loses one ring per second), without using an acutal super transformation, which includes the user going through a physical change. Gameplay-wise, the Super Shields have no impact on gameplay, except giving Knuckles and Tails new and/or improved movesets to combat Metal Overlord.
In Sonic Heroes, when Sonic transformed into Super Sonic, he transfered some of his energy to Knuckles and Tails. Instead of a complete super Transformation, they get a Super Shield that allowed them to fight on the same level as super transformed character. After the battle with Metal Overlord, Knuckles and Tails lost their Super Shields, as they possibly ran out of sufficient energy to be maintained, once the team powered down.
Strangely, only one Ring per second is used between the three members on Team Super Sonic, as opposed to three. This may be because that since the Super Shields were created by Super Sonic, they are maintained by his power alone, which allows only Super Sonic to burn through their shared Ring count instead of three people at once.
It's possible that the Super Shields are actually the modern designs for Super Tails and Super Knuckles, as there is a significant lack of differences between the Super Shields and their Super Forms from the classic games (especially Knuckles'). The in-game files' naming convention, and some of the dialogue during the final battle, seem to imply that this is the case.