It's a common saying that indicates time has value. To do work takes time, which is a limited resource that in the short term we all have the same amount of. The rich do not get 25hrs per day nor do the poor get only 23hrs. All of humanity gets a 24hr day.
What makes time valuable is its limited quantity. For instance, consider trading card collectors. They may spend thousands of dollars to buy a piece of paper/plastic. Why? Because of how rare that card is.
There isn't a definitive guide on what makes an activity meaningful. Instead, that choice is left to each individual. Some people might find value in activities that others don't. It's a difficult choice since over time, our interests or circumstances may change; what was once valued can become valueless. We can find ourselves regretting the choices we've made even though in the past, it was enjoyed.
We can't erase the past, and none of us are perfect. Still, we do well to consider our past interests and try to determine whether the time spent now will be valued later. It can help prevent future regret. We can also try switching our focus to how we felt in the past when dealing with regret. Did you enjoy it at the time? There's nothing wrong with changing interests, but recognize the value it held then. This can help to remove regret we have about time not being spent how we would currently do things.