Once you’ve reached engagement distance (5′, 10′ if they have a weapon with reach), if you attempt to disengage (or move through) an opponent as part of your movement phase, they gain the opportunity to make a free attack against you at that point (out of sequence). They gain +2 to hit, and you don’t get any applicable AAC bonus from a shield. This does not count as their action when they get to their melee phase.
If you move by multiple enemies at engagement distance, they each are eligible for the free attack. If multiple enemies move by you at engagement distance, you are eligible for multiple free attacks.
For exceptions, see “Retreat”.