The bottom bar shows you how much upkeep you are paying for how many supply points. If you need more points, you have to research the next level. However, the upkeep will increase (as a percentage of your total income (all 3)). So there is no point researching it too early as you will be reducing your income and not using the slots as you won't be able to buy the ships anyway. The flip side is you have heaps of income and no ships points free so cannot buy new ships either