Your Homebase Power Level (represented by the bold number in the top left corner of the menu) represents just how strong your
Team of heroes are, and it actually dictates what
Missions they are prepared to challenge. If one of the Missions Icons in your
Map is outlined in red, this indicates your Homebase Power Level is below the recommended requirement for that given
Mission. Even though this won't prevent you from playing a mission, it's basically a warning sign that you must first increase your Homebase Power Level before taking it on.
Your Homebase Power Level is influenced by four different measures known as F.O.R.T. Stats, representing Fortitude, Offense, Resistance and Tech. You can see an overview of all of your heroes' most essential stats by clicking on the Heroes Tab on your menu. You can even see a more detailed breakdown of your heroes' stats by clicking on the See All Stats Button in the bottom left corner of the Heroes Tab.
In order to increase each of these stats and thus further enhance your Homebase Power Level, there are a number of things you can do. For one, you can
Evolve Your Primary Hero or invite more powerful players to join your
Party. Placing Support and Tactical
Squad Bonus Heroes in your squad will also help increase your Homebase Power Level, as well as by purchasing and unlocking F.O.R.T. Nodes in the Skill Tree.
Finally, placing and swapping Survivors in your Survivor
Squads will also influence your Power Level. In order to assign Survivors to your Survivors Squads, click on the Squads Tab on the menu and then select Survivor Squads. Each Survivor
Squad influences different F.O.R.T. Stats: EMT Squad (Fortitude), Training
Team (Fortitude), Fire Team Alpha (Offense), Close Assault Squad (Offense), Scouting
Party (Resistance), Gadgeteers (Resistance), The Think Tank (Tech), Corps of Engineering (Tech). (You must first purchase and unlock each slot in the Survivor Squads in order to assign a Survivor to them.)