What does Buying a property below replacement cost mean?

[Matthew Metros]

The key to RE investing is having equity in my opinion. Buying below replacement cost in a growing market is a strategy that I hear many blogs and REI gurus talk about.

For example, I heard somebody say, "You want to buy below replacement cost (use 150 sf) in growing markets."

What do people mean when they say this? Is this a term that gets thrown around loosely?