Explain to me how "Trading Up" a house after a few years works?

[Jeff L.]

I've heard of this strategy where, instead of holding and renting a property for the length of the 30 year mortgage, you only keep it for a few years and then "trade up" to 2 properties or a better property. And you build wealth and your portfolio this way.

Can you explain how this works, preferably with some examples with numbers?