Devour is an ability which uses creatures to gain power on another creature. In other words, when a creature with devour comes into play, if the person who casted it wants to, they can sacrifice any number of creatures. Then the creature with devour comes into play with as many +1/+1 counters on it as were creatures sacrificed.
All devour creatures say "Devour 1" or "Devour 2," etc. When a creature such as Thorn-Thrash Viashino comes into play, something different happens. Thorn-Thrash Viashino has "Devour 2," which means that instead of just putting one +1/+1 counter on the creature for each creature sacrificed, you put two +1/+1 counters on the creature - double taking! And so on. If the creature has Devour 3, then you put three +1/+1 counters on your creature for each creature devoured.