“Locked” is a term used to describe a dreadlock or set of dreadlocks that are completely knotted with few to no sections of loose, undreaded hair. The dreadlocks at the bottom of the photo to left is an example of a “locked” dreadlock.
“Mature” is a term used to describe a condition of the dreadlocks which is usually reached somewhere between the dreadlocks’ age of 18 and 36 months (depending on the way they were started, how/if they’ve been maintained, the hair type, etc.) As you can see by the photo at above left, “mature” dreadlocks, in addition to being “locked” have a smooth, matted appearance and typically feel dense and solid when squeezed than younger locks. (Everyone’s hair locks and matures differently; some end up denser than others.) Mature dreadlocks typically have very few loose hairs on the surface and usually the only loose/undreaded hair is found at the scalp where all the newer growth is found.