When you tie a knot, you have two ends to keep track of: the standing line (running back to the reel) and the tag end (the short piece sticking out of the knot, usually a few inches to a foot of leftover material).
After tightening the knot, trim the tag end short — about 1/8 to 1/4 inch from the knot. Leave too much and it catches weeds or wakes water; trim too close and the knot can slip. Most knot instructions use "tag end" and "standing line" to indicate which strand to manipulate at each step.