Toot is both the act of blowing on a horn and the sound that comes out when you do. The word is fun to say, a favorite of children pretending to be cars. It suggests a kind of merry, or at least chipper, sensibility.
Speaking of merriment, toot can also refer to a night of drinking and carousing. Light, fun drinking: think 1920s flappers dancing in fountains, not hardcore alcoholics with bottles in paper bags. It's the car behind you letting you know the light has changed or the cheerful sound of a tugboat asking larger ships to make way. Even "tooting your own horn" is just not that serious — it's a lighthearted spin on the otherwise obnoxious act of bragging.