A 3PL receives your stock, checks it in, stores it by SKU and fulfills orders as they arrive from your store, a marketplace or wholesale customers. Most connect to platforms such as Shopify so orders flow in automatically and tracking flows back. Many also process returns.
Costs arrive in several pieces: receiving, storage (by bin, shelf or pallet), a pick fee per order and per item, packaging and postage, plus account minimums at some warehouses. Apparel adds complexity because every size and color is its own SKU, so even a small catalog can fill a lot of bins.
When comparing options, ask how they count SKUs, how quickly orders leave, how returns and damaged items are handled, and whether they can do custom packaging. A printer that also fulfills is a different model from a 3PL; the fulfillment options guide weighs them.