I didn’t quite understand how the knight acts as a joker.

CATAN Dice Game

After you have rolled the dice three times and “built” knights, you may turn any one of the dice so that its side with the resource that corresponds to the resource depicted below an activated knight is face up.

Example: You activated the “grain” knight in the last round and the “ore” knight in the round before; therefore, grain and ore are available for you as resource jokers. Now you have rolled ore, ore, grain, brick, brick, and gold. This allows you to convert one brick or gold into one ore and one brick or one gold into one grain. That way, you have three ore and two grain and also one brick or gold. You can use it to build a city.

In the following dice roll rounds, the “ore” joker and the “grain” joker are no longer at your disposition.