"As good as it gets" is a common idiom in American English. "It" refers to the current situation. The current situation is what it is - it won't (or can't) improve. The phrase can be used either literally or sardonically.
It's a beautiful Saturday afternoon. The skies are clear, the temperature is warm but not uncomfortable. You are lazing out in the sun in your backyard, there are steaks on the barbecue, and you've got the football game on your portable TV. You pop open an ice cold beer and sigh, "This is as good as it gets."
You've paid for what you thought was a luxury cruise vacation. The ship turns out be old, the accommodations are cramped, the food is barely edible. As the evening's entertainment - a man with an accordian - takes the stage, you complain to the management. "This isn't what the brochure described!" The manager shrugs and says "Sorry, this is as good as it gets."