Prestige Points

Prestige is a mechanic essential to the game - once you reach wave 50, you gain the ability to return to wave 1, with the process having both upsides and downsides. For starters, you get Prestige Points, a currency that can be spent to unlock and upgrade your Relics, which are very useful early on, and are what makes you return quickly to the point you were in previously.

Depending on the wave you prestige at, and some of your upgrades, you get more prestige points when prestiging. Note that these upgrades are multipliers applied to the number of prestige points you get at a certain wave, which is a number that increases (gets "updated") every wave beaten that's a multiple of 5 (5, 10, 25, 50, 65, etc.)

Additionally, the "repeated waves record" gets wiped, meaning you can start obtaining Essence and Experience again for waves you've beaten in previous prestiges, until you start repeating waves again during your current prestige (also called "run").

On the other hand, when prestiging, you lose all the gold you had in a run, both what you had left to spend, and what you spent on monsters to increase their tower level.

Don't feel discouraged if you "lose" a lot of progress if you have no option but to prestige - it's a healthy part of the progress you make in the game, and you should prestige often to make sure you're constantly increasing your numbers in any way.