Going to Ground

First off, what is going to ground, put simply, its sacrificing a turn of moving or shooting, to keep you head down when the bullets get going. Basically a better coversave. Particularly useful if you think that your opponent is going to target that unit. Only problem (other than the inherent no moving or shooting) is that, it makes your other units that much more of a target. So its best not to have that valuable unit go ducking down as soon as the first man falls. Wait until you are confident that your other men will be safe as well.

Now Guard have an even better thing goin for em, with their orders, instead of a +1 to the save, its +2 to the save, making even a lowly guardsmen as protected as a terminator. Devastating against any army that relies on shooting. But this coversave isn't cheap, it means two turns of inactivity. So in that case, its kind of an investment. you can use this in unique ways though. Want that platoon to be in perfect condition by turn three? You don't have to reserve them, just use orders.

Ultimately, a well timed +1 to your coversave as a Guard player, where your having one squad inactive isn't the end of the world can save your ass. Lastly, a lot of people don't know this, but you can initiate this after your opponent has rolled to wound, if they got really lucky, say 8 out of ten space marines land a wound, just go ahead and go to ground, and statistically only take half the wounds.