LÖVE calls 3 functions. First it calls love.load(). After that it calls love.update() and love.draw(), repeatedly in that order.
So: love.load -> love.update -> love.draw -> love.update -> love.draw -> love.update, etc.
Behind the scenes, LÖVE calls these functions, and we to create them, and fill them with code. This is what we call a callback.
LÖVE is made out of modules, love.graphics, love.audio, love.filesystem. There are about 15 modules, and each module focuses on 1 thing. Everything that you draw is done with love.graphics. And anything with sound is done with love.audio.