Your fan engagement score can give you a better idea of how well your Facebook page is engaging supporters.
It's calculated based on the average engagement each of your posts have received over the past two weeks. Then it weighs that against how many fans your page currently has.
In other words, it's not set in stone, and it can (and will) change according to the new content you post and how your supporters react. So keep track of your scores. Revisit them over time. Watch for any spikes and trends. This will help you better understand what works, as well as what doesn't.
If you're just starting out, a score of 50 is a great initial goal. Any score over 100 is quite an accomplishment, earning you the right to a well-deserved happy dance.