Before dry yeast can start fermenting, they need to absorb the water they lost during the drying process.
Yeasts are living organisms and rehydration temperature is critical for good yeast performance.
Fermentis recommends that top fermenting (ale) yeasts are rehydrated at a temperature between 25-29°C (77-84°F) and that bottom fermenting yeasts (lager) are rehydrated at a temperature range of 21-25°C (69-77°F).
Rehydration is done in a vessel outside the fermentor. The objective is to reduce the lag phase : the time necessary for the yeasts to start fermenting sugars to alcohol after inoculating the wort.
Rehydration is a simple procedure.
Rehydrate the yeast in 10 times its weight of water or wort. Gently stir. Allow a 30 minutes rest. Pitch the resultant cream in the fermenter.