3PL is short for third-party logistics, which is a fancy way of saying "a warehouse that stores other company's products, and when that company gets an order from a customer, the company tells the warehouse to ship the order to the customer". So the 3PL warehouse (such as Shipvine) is a "third party" (i.e., not related to) either the company or the customer. In many cases, the customer has no idea that the 3PL was even involved in the process. Instead of running their own warehouse, the company has outsourced the whole job to a third party warehouse.
For example, Susie sells custom keychains via a Shopify store. She used to ship them out of her basement, but her business keeps growing. Now she's faced with a choice: does she hire more people, rent some office/warehouse space, learn all the ins and outs of shipping, and continue to do it herself, or does she outsource all of that to a 3PL? She decides she'd rather focus on her business rather than reinvent the wheel, so she ships her products to Shipvine. Now, whenever someone places an order on her Shopify store, Shipvine gets notified automatically, ships the order, and writes back the tracking number. Susie can focus on running her business and providing customer service, not picking and packing.