TaB is a diet cola soft drink produced by the Coca-Cola Company. The Wayward Vagabond finds a large stash of the beverage stored in the second cabinet in the Skyship Base (which is, appropriately, opened by pressing the "tab" button on the keyboard). He seems very excited to find the drink, though this is likely only because he is so famished that any sort of food (or drink, in this case) would interest him. Indeed, as a calorie-free beverage, the drink serves little nutritional purpose and would do little to quell the Vagabond's hunger.
The cans of TaB, both empty and full, were naturalized into Can Town's growing population, which, being the egalitarian, civil society that it is, doesn't discriminate against cans based on their contents or their lack of such. In the Can Town game of industrial expansion/chess the TaB cans served as pawns for both sides, though tragically the drama and action of the game eventually lead to the untimely demise of one unfortunate citizen.
Wayward Vagabond later uses the drink, along with some Cans, to defuse the situation involving the Peregrine Mendicant and the Aimless Renegade. This prompts Aimless Renegade to deem the drink too warm, so he sets off to the Frog Temple to find Jade Harley's abandoned refrigerator to cool the beverage down.
Additionally, Feferi tries to drink Alternian TaB after a conversation with Eridan but fails with a GLUB since she's underwater. However, the can itself shows marvelous structural integrity by holding up to the extreme pressures of an underwater environment.
John finds a can in one of the mailboxes of his land and reacts with genuine excitement, since he is in need of a good sugar kick, and he expects it to be JAM PACKED full of real, actual sugar. It is likely due to the drink's complete lack of any such thing that he decides it sucks.
There appears to be another troll equivalent to TaB called the Alternian Soft Drink, which features Tavros-like horns on its sides and was used by the Condesce to take control of the market from the Subjugglators and their Faygo. Though the drink contained no sugar whatsoever, the Condesce sold billions of them to unknowing lowbloods - a fact that Meenah would have taken great pride in if she heard the story when finding it in a dream bubble chest. The Condesce is later seen with what an Earth version of TaB on her desk on Derse.