Glossary
Contest
In one line: On defense, going out to fight the attackers for an area instead of waiting on site.
What it means
Contesting means leaving your site to fight the attack before it arrives: in mid, in a main, in a passage. It takes space away from the enemy, gathers information and sometimes lands a surprise kill.
It is a choice, not a duty. Contesting makes sense when you have utility to fall back, a teammate covering you, or when the attack keeps leaving that area unchecked. Contesting alone and for no reason often hands the attackers the first kill and leaves your site one defender short.
An example
On Ascent two defenders set up in mid at the start of the round, one towards Market and one at Top Mid. If the attackers try to take mid they meet resistance there, with a smoke ready to fall back. The attack does not get mid for free.