The thumbnail version:
- Emulsion will absorb moisture from the air
- Too much moisture in the emulsion will cause problems
The full version:
A coated, unexposed screen will absorb moisture from the air because emulsion is hygroscopic (prone to absorbing moisture). If this happens you can run into problems with inconsistent exposure, ruined positive films, and premature breakdown.
Ideally, you should store your coated screens in a space such as, say, your darkroom where the humidity should be below 40%. And how can you keep tabs on the humidity? You need a hygrometer. A digital version can be bought from any hardware store for just a few dollars and will prove to be worth the minor investment many times over.
In more humid climates you may have to use a de-humidifier to keep the humidity down under 40%.