HP wash
HP Washing
Hp wash is using AP resets to switch points from mana to HP, and this can be done every level, though there is a limit to how much you can wash.
The reason why people would do this is because at later high levels, at bosses like Pink Bean and Horntail, which deal about 9-10k damage(Horntail deals 5-6k) Most range classes don't have enough hp naturally to survive, and even with HP equips, all maxed in slots with 10%, won't help.
So the only way to survive is HP wash, though it costs ALOT, and this isn't recommended unless your serious about playing a character, and won't quit, as it may cost hundreds of dollars to wash.
Alternatives
You can just add points from when you level to HP, as it's not really the same thing, but it costs alot less, but in turn your stats will be a little worse.
Also, you can AP reset from your AP points also(STR,DEX,INT,etc.) Though this ISN'T recommended ,as it's best to take the ap from MP, as it drains alot slower than HP, but if must, then it is done.
Once in a while, there are deals on AP resets, as last year there was an event where AP resets where 20 of them were only 20k nx, though don't count on it again, but it's best to watch out for these events.