Your health meter is a red strip that goes halfway around the circle on your HUD. Whenever you are hurt in any way, shape or form, the bar will decrease. Your health will refill itself over time, but in a firefight that can be inconvenient. To quickly regenerate your health, eat/drink items from Freckle Bitches.
The Stamina meter is below the health meter, and is yellow. There is no quick fix to low stamina, all you can do is stop running, and the bar will refill itself.
The Respect Bar is on the lower half of the circle, and has a little # on the right end of it, this determines how many missions/strongholds you can do before you need more respect. Each mission/stronghold takes 1 #. See Respect for ways to increase your Respect.
Cribs are houses that you earn in the main story, one for each gang. The Saints crib is a run down house near the bridge. The Vice King's crib is a penthouse near the High-End Retail District. Los Carnales crib is a mansion near the airport. The Rollerz crib is a mansion that is pretty much the northern-most point of Stilwater.
Strongholds are the central command posts for gangs in areas. Taking over strongholds takes over the area that the stronghold is in, earning you money every day for that area. Every area that you don't earn in the main missions, you must take the stronghold there to take over that territory.
A pushback is when a territory you have taken over, is underfire by the gang you took it from. Pushbacks stop income from coming from that territory. To stop pushbacks, you must go to the area in distress, and take out the 6 gang lieutenants there. You do not need to kill all of the gang members, just the lieutenants.