These are catch phrases we hear often, both talking about the concern of competition.
A moat is basically protection, something that protects your business from having others enter it.
Fast followers are those that.. well… follow you quickly, after you launch/start to gain popularity.
Having something that you can consider a “moat” that stops “fast followers” tends to help investors feel more safe and confident in their decision to invest in you.
Typically, this comes in the form of Intellectual Property (or “IP”).
This can be a social-construct like brand – that you’ve carved a ubiquitous name for your company/product. (See: Kleenex is practically the English word for tissue paper, but Amazon as the default choice for shopping works too)
This can be filing patents, protecting the business via law and litigation. (See: Pharmaceuticals like Amgen, or IBM for a tech example)
This can be R&D, that you’ve developed something very difficult to replicate in form or scale. (See: Google, AWS)
This can even be your business model/pricing – if you’ve secured a way to do this at a cost others can’t really compete at easily. (See: Tesla)
Truthfully, these moats are often not nearly as secure as investors believe. Bad press can destroy a brand. Patents can be beaten with enough money to fight in courts. R&D is often never as hard as anyone thinks, once it exists it’s repeatable. Business deals, if had, can be had again.
Nevertheless, investors are taking a risk on your startup – it’s your duty to do anything you can do to protect their investment. Try to develop moats, protect against fast-followers, and elucidate how you’ve done so to your investors (or potential investors)!