Glossary
Eco round
In one line: The round your team saves in: you play with pistols and little else, while still trying to gain something.
What it means
The eco round is the round where you decided to save. It is not a round to give away: pistols can still win it, and even when you lose you can gain something: a kill that picks up a rifle, a plant that gives everyone 300 credits, information about the defense's habits.
On an eco you play together and up close: pistols and cheap guns work at short range, so you look for tight angles and fight as a group, hoping to pick up the enemy's guns.
An example
You are on an eco on attack. Instead of spreading out, all five of you hit the same choke and fight up close. You take two of them and pick up a Vandal and a Phantom: even if you lose the round, the next one gets easier.