Glossary
Eco
In one line: Spending little or nothing in a round so you can buy properly in the next one.
What it means
Going eco means saving: you buy at most a pistol or light armor and keep the rest of your credits. You do it when the team's money is not enough for full guns and armor: better to lose a round you would probably lose anyway and arrive at the next one with rifles and utility, than to half-buy two rounds in a row.
An eco only works if the whole team does it together. If two players buy and three do not, you lose the round and nobody can afford a full buy next round either.
An example
You lost the pistol round and round two. You have about 2000 credits each: buying now gets you neither rifles nor heavy armor. You all eco together, and in round four everyone buys rifles, armor and abilities.