Toner is a fast-penetrating liquid that delivers the skin a quick hit of hydration and helps remove some dead skin cells off the skin's surface. Toner is like a primer for the rest of your skincare routine, like serums and moisturizers.
Think of a toner as a "prep" for your pores- it brings your skin back to its natural acidic state, removes impurities, and helps your skin absorb your skincare products. Think of your skin as a dried-up sponge. Putting your serum or moisturizer on a dried, brittle sponge won't accept it and will not absorb as quickly. But if you wet the sponge, the serum or moisturizer penetrates much more rapidly and deeper.