In short, the 3-1-1 rule is: Each liquid you bring through the TSA checkpoint must be in a 3.4-ounce or smaller container ("3"), all containers must be placed inside one clear quart-size plastic bag ("1") and each passenger is only allowed one plastic bag ("1").
Why does the liquid rule exist?
The in-flight liquid limit was introduced in 2006 after British police foiled an Islamist terror plot to detonate explosives on transatlantic flights. They planned to smuggle liquid explosives disguised as soft drinks in their hand luggage, in what would have been the deadliest terror attack since 9/11.
Does TSA care about quart size bags?
You are allowed to bring a quart-sized bag of liquids, aerosols, gels, creams and pastes in your carry-on bag and through the checkpoint. These are limited to travel-sized containers that are 3.4 ounces (100 milliliters) or less per item.