The word itself is Italian and means the tumbler that is used in bingo and raffles.
By poetic extension, it is used the way Americans say "crapshoot' or "the luck of the draw' or lottery or 'numbers game'
The song "La Vida Es Una Tómbola" by Marisol and another version, for Diego Maradona, by Manu Chao is the most famous example. (Life is a raffle)
There's a line in "Evita" that goes: "She won't help the hungry once a month at your tómbolas / she'll simply take control as / you disappear" (It's from "The Actress Hasn't Learned the Lines").