What is a page view? Is it a person that looks at my form on my page but doesn't fill out any info.

  If I upgrade can I downgrade back to the free option at a later date?  If so what is the time frame?

A page view is any time the form is loaded.  If the form is embedded to a web page, each time the page time loaded, the form will also be loaded. 

If the form is embedded to a high traffic page, to reduce the number of view, replace the embedded form with a link to the form.  That way, only the user who wish to fill out the form are viewing it.

As for the subscription question, subscriptions can be cancelled at any time.  If you only need upgraded limits for a month, upgrade for a month then cancel immediately.  You will retain the subscription benefits until the end of the paid period and be automatically set back to a Free account after that.