A font is neutral square on a map which provides nora, deployment zone and font bonus zone for the player (or team) that controls it. Each font begins neutral, and comes under a player's control when that player has a real champion in its deployment zone, and this deployment zone is free from enemy units, including opposing relics. (Once the font is under your control, champions are free to leave the font zone.) Note you can capture opposing fonts in the same way you can control neutral fonts.
Fonts can't be destroyed or attacked (though sometimes they will appear to be affected by various abilities).
A font which has units from both players (or both teams) is contested, and turns back to being neutral until one side controls it. Fonts in the middle of the map are commonly contested while the two sides fight it out. To contest a font you must place a relic or move a champion in to the font zone.
Each font generates 12 nora and the player recieves this at the beginning of their turn for each controlled font.