"sticky" refers to a post. When you start a post, you can choose to make it "sticky" or not. If it is "sticky" it always stays at the top of the list of posts, even if there are other posts that are more current that would normally be at the top of the list. It's usually for important topics and announcements that you would make a sticky post.
A sticky is a post that will always stay at the top of your groups forum. You can only add a sticky post if you're a Group Owner or Admin. Most of your posts will be marked "not sticky" which means they are just a regular post.
Here's how to add a sticky post in a group:
Mouseover the 'Groups' tab.
Click on 'My Groups' in the drop-down menu.
Go to your group's homepage.
Click the 'Add a Post' button at the bottom of the page.
After you create the title and the post, at the bottom you will see: 'What is the status of this post?'
Select 'Sticky' from the drop-down menu.