In the simplest terms, there are two types of edges:
Hard Edges are when value or color transitions abruptly.
Soft Edges are when value or color transitions gradually.
In the real world, lines very rarely exist; there are only really edges. Therefore, if you can master edges, in any medium, you can master that medium.
In watercolor, there’s one way to make a hard edge: simply lay a stroke of wet paint on dry paper, and voila! You have a hard edge.
Soft edges have more options. One way is to lay down a patch of color, and then pull the pigment out with some clean water.
Another method is to begin with a wash of clean water, and then drop the pigment into it. The pigment will spread via capillary effect and give you a nice, soft transition.