Level abilities count the number of level counters on the creature. Hexdrinker starts with zero level counters. Once it has at least three but no more than seven level counters, it’s 4/4 and has protection from instants. Once it has eight or more level counters, it’s 6/6 and has protection from everything. It always has level up {1}.

Protection from everything means that Hexdrinker can’t be the target of any spells or abilities, it can’t be blocked, nothing can be attached to it, and all damage that would be dealt to it is prevented. It can still be affected in other ways (such as by Winds of Abandon cast for its overload cost, for example).

Protection from instants means that Hexdrinker can’t be the target of instant spells or abilities from instant cards (such as an ability of an instant card that triggers when that card is cycled), and all damage that an instant spell or instant card would deal to it is prevented. Nothing other than these events is prevented or illegal.

If an effect has set Hexdrinker’s power and/or toughness to a specific value after it entered the battlefield, leveling up won’t change that characteristic.