1. To not push against prevailing behavior/norms/attitudes, occassionally including bowing to peer pressure.
2. To not attempt to exert a large amount of influence on the course of events, whether a specific series of events or events in general. A person who does this is often referred to as "laidback" or "easygoing".
Past tense: "went with the flow"
2. Mike really wanted to ask Jane on a date, but the timing was off, so he went with the flow.
To 'go with the flow' means to not have a particularly strong opinion on something, and thus follow the majority.
==> First known to be used by the Roman Emperor Marcus Arelius (yes, the one from Gladiator) in his writings 'the Meditations.' Marcus wrote a lot about the flow of thoughts and happiness and he concluded that "most things flow naturally" and in his opinion it was better to "Go with the Flow" than to try and change society.
MAN: "Yo dude, what's up?"
DUDE: "Nothing, I was just wondering why you weren't being TOTALLY EXTREME like I am today, with my cool anti-fit jeans and my tight fushcia mohawk!"
MAN: "I dunno, dude. I just thought I'd go with the flow rather than dress like a total dickhead, y'know..."
DUDE: "Yeah, I know what you mean, man."