Elemental threshold, often shortened to simply "threshold," describes how much elemental affinity is needed to cast most spells or use some abilities. Elemental affinity is most often provided by sites, but can come from other sources as well. For example, provides one fire affinity 
, denoted by the red triangle near the bottom right of the card. Similarly, is a minion that provides fire affinity.
There are four elemental thresholds:
, e.g. 
, e.g. , e.g. 
, e.g. Unlike mana, you don't spend your affinity. It is merely a requirement you must meet to cast spells and use ability. For example, to cast with its three fire threshold , you must have three or more fire affinity. Some spells, often artifacts, have no threshold requirements to cast.