Please explain how refinancing keeps your investments rolling

[Jeff L.]

As a new investor, I'm reading a lot about lending but I don't fully understand the concept of refis. I often hear people here say something like "...then I'll refinance to keep my investments rolling."

1) When people say that are they typically talking about cash out refis?

2) Do refis count as one of your "4" mortgages that banks will let you have at once?

3) Does a refi benefit me at all if there's no rehab or appreciation involved, or is it only relevant if there is a rehab or appreciation? Say I purchase a move-in ready house for $100k putting 20% down, and there isn't any appreciation for the foreseeable future. How does refinancing at any point help "keep my investments rolling"?