Upkeep is the maintenance cost you must pay daily on every ship you own. Fleeted ships incur additional upkeep for their crew, which does not apply to ungrouped vessels at a colony.

If you have insufficient credits to pay for your upkeep then some crew will mutiny and take their ship with them.

You can see your current ship upkeep together with other running expenses on the Overview page of the Command menu. The Shipyard interface also shows your current upkeep.