Power = percentage of bosses health dealt as damage in the raid time limit if you have 6 of these in your party.
So if your power is above 100% then you can defeat the boss in the time limit. More than 100% means you've got more room for error essentially, because if it's 100% exactly then you can just about do it, but if the power is 200% that means in theory your 6 Pokemon could defeat that boss twice in the time limit of a single raid.
In higher level raids that cannot be soloed think of power as total damage done. Time to win is a measure of DPS - higher DPS means the boss faints quicker, but isn't necessarily the better choice because you've got to balance out total damage and DPS when you've got a limit on the Pokemon that you can bring, and consider DPS loss due to rejoining because you have to select your Pokemon again even with a party and also load back into the raid, so bringing 20 glass cannons isn't necessarily the best idea in practice even though in theory it is.
For example Absol has a simulated clear time of 711s vs a PC/P Mewtwo whereas Tyranitar has a time of 718s. In theory that means Absol does more DPS but if you don't have 10 Absols that you can throw into the grinder against Mewtwo that time is meaningless, as that difference in time is less than 1% but a single Tyranitar will deal much more total damage than a single Absol.
Checking the moveset's combat logs will show you their simulated DPS as well as total damage output before fainting though that'll vary even for the same Pokemon depending on the boss' charge move usage which is random.
Using the PC/P Mewtwo example again, an Absol will deal somewhere between 17-18 DPS on average and Tyranitar will also deal around the same, but Tyranitar does much more damage per life.
