Aeration
In the phase immediately after the yeast is added to the sweet wort, it needs to multiply to ensure a good fermentation. To do this effectively it needs oxygen.
Cooling the wort
If yeast is pitched when the wort is too hot, at worst the yeast will be killed and at best it will produce off flavours or foam out of the fermenter, or both.
Sparging techniques
Sparging is the rinsing of the grain at the end of the mash to remove the last of the sweet wort.
Mash techniques
Depending on what type of beer you're brewing and the malts you're using, you may need or want to employ a different mashing technique to the most common type, which is a single infusion mash. If you're new to all-grain brewing, stick to single-infusion mashing while you get other aspects of your brewing perfected.