ASSUMING you have 1 gate at an airport, there are at that airport 10 available "slots" (times that gate can be used in a day). if you have a route/plane combination that can go 11 frequency, but only buy 1 gate at each airport, you'll see a * meaning you could increase your frequency but you don't have enough space ("slots") at one or both of the airports.
Whichever airport has the lowest number of free slots determines that limit, so if you have a plane leaving your starting city using 4 "slots" at it, you have 6 slots free remaining from that gate. 6 would be the maximum frequency between the two even if the destination has 10 slots free.
renting an additional gate at whichever airport has the least slots available can raise, or eliminate any slot constraints (limits due to gates). if they're both tied at slots free you need to rent another gate at each airport to have the same effect.
(i hope that makes sense).
