Glazing (or layering) is the term for painting on dry paint. Usually to create darker, richer values or deliberately create shifts in hue.
Due to the transparent nature of watercolours the colour we apply on top won't completely cover the underlying colour, but will create a mix out of the two.
If not done with care, this can create
Now, to avoid some of these problems you could learn everything about the theory and pigment properties like transparency, staining quality, colour theory, etc.
Or as I prefer, through trial and error.
A simple but effective exercise I use to learn about how my colours mix when layered is to paint small glazing swatches :
Here I used the same 2 colours and reversed the layers. For example Hansa Yellow first and Ultramarine on top, then the other way round. Depending on which colour goes first the resulting mix will be different. Compare that to the strip underneath which is the colour when the 2 pigments are mixed in the palette.
Choose any combination you like and practice creating secondary colours (purple, orange, green) or some unusual ones, like dark mixes for shadows and earthy colours.
Pro tip: Layering transparent colours (opposed to opaque or semi-opaque colours) will create darker values.
And hopefully you'll avoid muddy colours and ugly mixes in your paintings!