Stock foregrip posts aren't always where you need them to be, since they're one-size-fits-most. But adding or moving a foregrip to a different place on a rail can make your marker easier to hold and stabilize during firing, which will make it easier to hit your targets on the move.
Foregrips come in two basic varieties, angled and vertical. Vertical foregrips are for holding your off hand essentially in front of your dominant hand in steadying the marker, in the same basic position. An angled grip is designed so you can hold the barrel comfortably in your palm. Which one is more comfortable for you and fits your playstyle is a matter of preference.
Many foregrips come with the ability to fold against the barrel. This moves it out of your way when you aren't using it, which can be especially useful when firing prone or lining up a longer shot.
Some trigger frames are also called grips. These are usually part of an electronic trigger or response trigger upgrade.