Move accuracy refers to the probability of a move hits, from a number between 0 and 100. The exact probability also depends on the user's accuracy stat and the target's evasion stat. By default, these stats are equal to one and do not exist outside of battle.
An example is that Fire Blast has an accuracy rating of 85, meaning it has an 85% chance to hit and a 15% chance to miss. Note that an accuracy rating of 100, as seen in moves such as Thunderbolt, is not equivalent to the property of ignoring accuracy and evasion checks to never miss, as seen in Aerial Ace. While neither kind of move will ever miss under default accuracy and evasion settings, the first kind can still miss if the user's accuracy is less than one or if the target's evasion is greater than one.
The game uses a random number generator to generate a random integer between 1 and 100, inclusive, to decide if a move hits or misses. If the randomly generated integer is less than or equal to the effective accuracy of the move after accounting for accuracy/evasion modifiers, if any, the move hits.
Moves with high damage power (such as Focus Blast) or have powerful side effects (such as guaranteed paralysis in the case of Zap Cannon) often have relatively low accuracy to compensate for their powerful effects. Moves that knock out the target in one hit (namely Sheer Cold, Guillotine, Fissure, and Horn Drill) have an abysmal accuracy rating of only 30% specifically to make it much harder to ease through battles with them.