Flux is a material that is used to help solder metals together. It can be in the form of a liquid, paste, or powder. Flux allows soldering to be done at lower temperatures by preventing oxidation of the metals being joined.
Flux can dry out over time if it is exposed to air. Once flux dries out, it can no longer be used for soldering. To prevent flux from drying out, it should be stored in an airtight container.