This is the easy one, but it's also the one that needs a clear understanding. "Clicks" on a post are different depending on what type of post it is. Any link associated with the post counts as a click, so whether they click the name of the page, the date, the link if it's a link post, the play button on a video, the image itself on an image post, or the "more" button to expose more of a status update, it counts.
This number has to be viewed in relation to other posts of its kind. For example, getting 50 clicks on a link to your website is more impressive than getting 60 clicks to an image because images get clicked a lot more often than links. Comparing how links perform relative to other links is the right way to look at these numbers.
Likes, Comments, and Shares
This is a no-brainer. Did they engage? Did they find enough entertainment or informative value in your post to do something publicly with it?
If you make statements, you should get likes from those who agree. If you ask questions, you should get comments from people replying. If it's an image, are people sharing it? These are the pieces of information that you'll want to track and improve upon when viewing this number.
This is the all-important number of the group. Regardless of how many people are interacting with your content or clicking through to your posts or links, are enough people seeing it? The previous two numbers have an influence on reach, but they are usually superseded by the advertising dollars spent.
If you're advertising, the previous two numbers are still extremely important. Throwing money at bad posts will get you a short-term gain, but it isn't sustainable. If your advertised posts are not able to get the positive algorithmic benefits of engagement and clicks, your dollars are going to start yielding less. Thankfully, the opposite is true as well.