Build is a keyword that will appear on minions followed by a number, for example - Build: (2)
When you summon a minion with the Build keyword, it comes into play as a 0/10 Structure. At the beginning of your turn, your minion's build counter decrements by one: this occurs every turn until the build counter reaches zero. Once the build counter reaches zero, the Structure will transform into the minion you played from your action bar! At this point, the minion is fully active and can begin to move and attack.
While a minion is "Building," you cannot dispel the fact that it is building and will transform at the end of its build countdown. This means that you can treat buildings as regular Structures - you can target them with spells, heal them, damage them, transform them, destroy them, etc. They will however eventually transform into their final form if not destroyed or removed.
If a Build Structure is returned to the action bar, the owner will receive a copy of the 0/10 Build Structure. Buildings will match their minion's Mana Cost, allowing you to play the Building again for its original cost, and the structure will eventually build into the specified minion.
Any buffs that are applied to a Build minion while the card is in your action bar will apply to the final/transformed version after it is done Building (e.g. via Phantasm) And finally, a "Building" is considered a token, so it is not a viable target for revive effects (you can, however, revive the final product).