What is a "window?"

[[Ca...]]

I have seen this term a lot and am not sure what it means. Would love clarification and maybe some examples?? Is my one-day-good out of three-days-bad a window? And what do they mean about whether or not you are getting better?

[[Be...]]

A window is when you feel relatively good/normal. Very good sign 

[[Ca...]]

Thanks - it is beginning to feel like a window where I press my nose against the glass and wish I were permanently on that lovely other side...