Pre-attack and post-attack abilities are effects that take place when an attack card with either symbol is played. Both abilities are mandatory. If a card instructs a player to resolve a or ability, then it must be fully resolved before moving on to the next step of the attack.
The key difference between the two is that a player may play a card with a pre-attack ability, and then choose not to attack, whereas a post-attack ability is only ever resolved after an attack is made. Note that these abilities can be part of a reaction card.
Note that many keywords have numerical values, so can be legally resolved by choosing 0, effectively cancelling the effect.