A cart is a cartridge - a plastic and stuffing piece that pushes onto or into a separate atomizer. You can refill them, you can rinse the cart and stuffing and dry the cart, and pat dry and refluff the stuffing and put it back in. If you have a 510 you can replace the stuffing with the precut blue foam stuffing from strictlyejuice.com. If you got something smaller than a 510 you can cut down those little stuffing pieces for your carts. If you have something bigger, you can cut stuffing from the bulk blue foam whihc is Marineland Rite-Size U Bonded Foam Sleeve for H.O.T. Magnum Filter Canisters (an aquarium product Petsmart carries).
But most people switch to cartomizers for most of their vaping. Hold more liquid, more consistent way to vape, but they have limited lives. We call them cartos. They cannot be restuffed. We keep them damp with suitable liquid to get 4-7 days use out of each or more.