A thin
edge of a different colour around a region of a
flag or
shield, or a very thin
stripe separating two regions. The term is sometimes regarded as inappropriate in
heraldry, which prefers bold areas of colour.
Whenever you depict regions you inevitably have lines drawn separating them, which in theory don't have any thickness or colour, though in practice they are drawn in black; but these are not fimbriations.
A fimbration has slight but genuine thickness. It is not merely an accident of drawing but a real part of the design.
Thanks to JudyT for pointing out that my original familiar example, the white edging around the red crosses in the Union Jack, is not a true fimbriation, because the white is actually the complete background of two flags, merely voided or covered up by other flag regions superimposed over them. A real example is the white around the blue cross of the Norwegian flag, separating it from the red background; and conversely with the Icelandic flag. It is not simply a thin stripe: those of the US flag aren't fimbriations.
Two regions are said to be fimbriated such-and-such a colour if they have such a fimbriation between them. The word comes from the Latin for fringe: see also fimbria.