ICSP stands for In Circuit Serial Programming ... This is a good comparison to an Arduino shield as it allows you to quickly add functionality of your Arduino with not much thought.

"The Esplora is 100% Arduino software compatible but doesn’t provide Arduino Shield connectors (i.e. it ... a micro USB connection, an ICSP header, and a reset button. It contains everything ...

Sure, you can add tons of functionality via strap-on boards called “shields”.. but those can get expensive quick. Want your Arduino project to be able to play sounds? That’ll be $20.