First of all he is Castiel - The Angel of the Lord from the Supernatural TV Series
This is how how he look like - Castiel and the actor :P
In what context this Castiel said it?
It is from episode 5.13 - The Song Remains The Same
(Also a title of Led Zeppelin song by the same name)
In this episode Anna (another angel) escape from heaven (because she's been notty angel so they put her in prison heaven), now she has escaped. She want to kill Sam so that all the apocalypse business will finish. So she decide to go back in time at 1978 before Sam was born and kill their mom - Mary Winchester.
Dean ask Castiel to take he and his brother back in time. So because Castiel has been a bit notty also, he is not indirect contact with Heaven Host, so no power recharge, and very hard to take passengers back in time.
SAM : Why nineteen seventy-eight? I wasn't even born yet.
CASTIEL: You won't be if she kills your parents.
SAM : What?
CASTIEL walks up to DEAN and SAM.
CASTIEL : Anna can't get to you because of me. So she's going after them.
DEAN : Take us back right now.
CASTIEL : And deliver you right to Anna? I should go alone.
DEAN : They're our parents. Cas, we're going.
CASTIE : It's not that easy.
CASTIEL walks a few steps away.
SAM : Why not?
CASTIEL : Time travel was difficult even with the powers of heaven at my disposal.
SAM : Which got cut off.
DEAN : So. what, you're like a Delorean without enough plutonium?
CASTIEL : I don't understand that reference. But I'm telling you, taking this trip, with passengers no less.
**Ok for those who also don't understand Dean's reference is about the Michael J. Fox's movie - Back to the future. Delorean is 'the car that can go back in time' and plutonium is its fuel.. somethime like that. So what Dean meant about Cas he is like a the car that does not have enough fuel. And Cas is an Angel who obviously not accustomed to human culture so he don't get the movie reference that Dean is referring
