Extra adhesive that gets forced out of a joint once clamping pressure is applied. Squeeze out serves as a good general indicator that you have enough glue in the joint. Some folks will clean up squeeze out using a damp rag or sponge, but this tends to drive the adhesive into the wood grain, which can cause staining/finishing issues later. Others will scrape the glue off once it dries, but this can pull wood fibers off the surface. My preferred method is to wait for the glue to skin over (about 10-15 minutes), then carefully scrape the glue off using a card scraper.