When situations could depend on a Characters ability for an effect the Character must make an Ability Save or Check. The Game Host will tell the player which Ability is being checked or in most cases it will be said in the description of the Action prior to the roll.
Saves and Checks are determined on a d25 vs. the Ability. The player will roll a d25 and then post what the Ability is and what score the Ability has. If the result is equal to or less than the Ability then the Save/Check was successful. If its higher than the score then the Ability has failed.
Example:
Valen is crossing a rope bridge during a high wind. The bridge begins to sway so much that its possible someone could lose their balance and topple over the side. The Game Host calls for Valen to make an Agility Save to keep their balance while crossing the bridge.
Valen rolls for the save...
d25 [15 Agility] and the number shown after the die is rolled is 12.
Valen has succeeded in keeping his balance and continues to cross the bridge.