Glossary
Bonus round
In one line: The round after a win where the team keeps the previous round's guns and buys only what is missing.
What it means
After winning a round with cheap guns (usually the pistol round and round two), the survivors still have their weapons. In the "bonus" round the team chooses to play those weapons instead of buying new ones, so they arrive at the next round with more money.
It is a gamble: win the bonus too and you are three rounds up with a strong economy; lose it and the enemy got their recovery round while you saved. The call depends on how many guns survived and how strong the enemy economy is.
An example
You won the pistol and round two with a Spectre and Stingers. In round three you can afford rifles, but three of you still have SMGs: you keep them, buy only armor and play the bonus. Whoever loses their gun buys a rifle in round four.