The gear symbol looks like two gears turning, one bigger than the other, in unison.
When commanded to move to a new location, a Blockhead will pause as the game calculates its path. During this time, the gear symbol will appear and animate above its head. Once the calculations are complete, the symbol will disappear and the Blockhead will either begin moving or the destination selection will be aborted as inaccessible.
Very long or complex travel requests may abort due to internal limits on the game's path calculator. The gear doesn't always appear as the calculations for short distances happen too quickly.