Definition: a thin sugar-and-citrus syrup spooned over a warm cake so it soaks in, adding flavour and keeping it moist.
A drizzle is the difference between a nice lemon cake and one that’s still lovely three days later. You mix citrus juice with sugar (not fully dissolved — a little crunch is part of the charm) and spoon it over the spongewhile it’s still warm, so it drinks the syrup straight in. As well as a bright hit of flavour, that moisture keeps the crumb soft for days. Prick the top with a skewer first if you want it to sink in faster.
In practice: drizzle while warm, in the tin — cold cake just sits under a puddle.