Escort is a radar jammer ship. They are used to make your units invisible to enemy radar, useful for planting traps and moving your fleet under cover. However, be aware that jammers do not make your units invisible, your enemy will spot them once they are within line of sight.
Combining a cloaked unit with a jammer makes a given unit totally invisible.
Usually, they are used to support Missile Cruisers, allowing the cruisers to sneak in close to the enemy’s shores. This lets you get within missile range of their base without being spotted, and unleash a lethal barrage of missiles.