Rehydrating dry yeast
It is usually recommended to "rehydrate" your dry yeast prior to pitching. Rehydrating dry yeast is easy and can improve the quality of your beer, which as home brewers, is usually the goal.
While not required, rehydration gently rehydrates the yeast cells themselves prior to being added to the wort. The events of rehydration in sterile (pre-boiled) water allow the yeast cells to start the aerobic portion of fermentation much healthier (##add description of yeast cell rehydration process##).
It should be noted that some yeast companies recommend rehydration of their dry yeast, while others do not. It is therefore important to consult the specification sheet for a particular yeast to find out if it should be rehydrated before use. However, this may be a marketing ploy to show how "simple" brewing with dry yeast can be (i.e. open packet, sprinkle on wort). (##reference description of yeast cell rehydration process here##)).
To rehydrate your dry yeast, simply sprinkle the dry yeast into a cup (1c) of 104°F sterile (pre-boiled) water 30 minutes before pitching. The temperature and time are recommendations, so RDWHAHB.