The parameter "Fleet strength" appears with the achievement of at least one of your ships of the maximum level of experience (translated into the status of "elite"). Each elite ship listed as yours (not sold) adds one unit to Fleet strength, which is luck points that increase the chance of getting a second container for free.
In other words, if you have several ships upgraded to "elite" status and/or you have ships acquired for Galactic Standards or DLC, which are elite by default, your Fleet strength will be equal to the number of such ships.
When selling an elite ship, the power of the fleet will decrease.
You can see this parameter in Pilot Profile.