Waste not, want not.
Basically, it means that if you use something sparingly, or without using it excessively, then you may not deplete or overuse it to the extent of it becoming useless.
For example: Money.
If one thinks about money as a resource, one needs to use it wisely and only willingly when needed. Frugality with the use of such resource is the most important. With such frugality in place, needing to manage and maintaining one's supply of money may never be a worry or an issue in the future due to wise use. Wise use, in this case, can be attested to only using money when needed and never wasting; hence, the proverb "Waste not, want not."