What is a flight level?
A flight level is an altitude expressed in hundreds of feet based on a standard pressure setting (29.92 inHg / 1013.25 hPa). FL350 means 35,000 feet. FL240 means 24,000 feet.
Why not just use feet?
Below 18,000 feet, pilots set their altimeters to the local barometric pressure. This gives accurate height above sea level. But pressure varies between locations - two aircraft at the "same" altitude could actually be at different heights if they're using different pressure settings.
Above 18,000 feet (the transition altitude in the US), everyone uses the same standard pressure. The actual height doesn't matter as much - what matters is that all aircraft at FL350 are at the same pressure altitude and therefore separated from aircraft at FL340 or FL360.
Semicircular rule
Aircraft heading east (0-179 degrees) fly at odd flight levels: FL310, FL330, FL350, FL370. Aircraft heading west (180-359 degrees) fly at even flight levels: FL320, FL340, FL360, FL380. This provides built-in separation for opposite-direction traffic.
In radarcontrol.io
Use cfl350 or c350 to assign a flight level. The sim automatically applies the semicircular rule when spawning aircraft. Below FL180, altitudes are in feet (c12000ft).