Potion ingredients are available for student use from the Student Store cupboard. Students also buy their own ingredients as part of their school supplies, and purchase additional materials and refills every year. Many witches and wizards buy their potion-making supplies at the Apothecary in Diagon Alley. Snape maintains his own private stores which include ingredients not available in the Student Store Cupboard, including boomslang skin and gillyweed.
Exact effect unknown, but presumably invigorates the user
The fifth years had to brew an Invigoration Draught, which Harry found rather easy to do because Snape was pretending that Harry was invisible in retaliation for Harry viewing the scene in the Pensieve. Harry did just fine, but Snape intentionally dropped and broke the flask, so Harry received a zero for the class (OP29).