Each player starts out with a max of 10 power. When you start a faction in the free world you will be able to claim up to 10 pieces of land (One per power) And as long as you lose no power no player can claim over these pieces of land due do your power. You lose 4 power upon death and if you go below the number of land pieces claimed (eg you have 6 power but 7 land) then other factions can claim over 1 piece of your factions land (a 16x16 square). You gain 1 power every 5 minutes of activity (I believe) For example 20 minutes of mining will gain you back the four power you lost upon death. For every new member that joins your faction it will gain their power so if you have 5 members all with max power your faction power will be 50. Be careful though not to let people join with low or negative power as this will put your faction power down and leave you vulnerable to other factions claiming over your land if it goes below your land claims. Hope to have helped.
As said above type /f power (Must be outside spawn area) eg free or end world. And to see another factions power or your factions power type /f who <faction name> Goodluck.