See: Holding Period.
The purpose of investment is to earn a return. You buy low and sell high, allowing you to book a profit.
Often, that return is viewed in distinct time chunks, especially when reported by a fund or when reviewing an investment portfolio. Typically, you might look at your quarterly return or your annual return.
A holding period return takes a look at the entire time an asset's been held.
You buy a rare coin in 2011. It's a quarter that features a mis-strike making it look like George Washington has a neck tattoo that says "THUG." You buy it for $200. You hold it for 8 years, deciding to sell it in 2019. You get $450 for it.
Your holding period return is $250 on the $200 initial investment, or a percentage return of 125%.