Credit

From Galactic Fleets Wiki

Credits are the life blood of our economy. They may not be good to produce ships or buildings, but a resource value is judged by the amount of credits it costs. With out this common bar to judge the value of things we would not be able to accurately or efficiently move goods around. It is the credits that control the stability of our lives and with them we can be assured a accurate estimate on the value of our goods.

Credits are generated as Tax Income from the Citizens on your Planets. You could be self sufficient and then Credits would not be needed but if you wish to buy Items, Troops, Starships, Planets etc from other Players then you will either need to trade or use Credits. It's a supply and demand economy so the value of nothing is set and the prices will fluctuate based on demand.

Your credits are put into your Fleet Fund and you will keep them unless you go inactive or violate the terms of play for Galactic Fleets and get banned / reset.