In a full flow oil filter the oil flows from the pan to the pump, through the filter, and out to the bearings. In a bypass system the filter tees into the line after the pump and returns the oil it filters back to the pan without passing through the bearings. It still cleans the oil but doesn't offer the same level of protection to the bearings as the full flow does. On the plus side, a bypass filter can be made to capture very small particles since it doesn't have to flow very much oil. Some large marine and truck diesels use both full flow and bypass oil filters to clean the soot out of the oil and greatly extend the oil life.
