Plastisol is the default ink in most T-shirt screen printing. It has no water or solvent to evaporate, so it can sit in a screen for hours without clogging the mesh. It sets by heat alone: the whole ink layer, top to bottom, has to reach the temperature on the maker's data sheet, typically around 320°F (160°C) for standard formulas and lower for low-cure ones, before the resin fuses into a flexible film.
Because it rests on top of the fibers instead of soaking in, plastisol gives bright, opaque color on dark garments and crisp edges. The hand is smooth and slightly rubbery, and it gets heavier as the deposit gets thicker. Puff, high-density, glitter and metallic effects are commonly built on plastisol bases.
Two things worth asking a printer: whether the ink line is phthalate-free, if your buyers or retailers care, and how cure is checked (a stretch or wash test). Under-cured plastisol looks perfect on day one, then cracks or peels in the wash. For the full trade-off, read plastisol vs water-based ink.