Hollow Mask

[SmaileyFace]

Can anyone please explain what this does?

[iceblademush]

It resets your level to 0, but you keep all the unlocked skills. So you basically can double up on skill points, as when you reset, you will very get back to the same level as you earning tons of XP.

[Alketal]

iceblademush said everything right.
On my own I'll add that this is still a mask, so its use under normal conditions conflicts with the use of other masks. Therefore, I personally see the benefits of this mask only in two cases:

1. A build that isn't built around the effect of a particular mask, but clearly benefits from doubling the SP. For example, a "Lump of Mana" build without "Bonfire flame", but with investments in all 3 skill trees.
   
2. Duplicity Shard - begins to appear somwhere around the 15th Galaxy, the special effect of this Shard allows you to use 2 masks at the same time.

In both cases, to get benefits from this mask you need to have some significant game progress - but it also means, that at this point you will be pretty ok without this mask. So, in my opinion, it has questionable utility.