How does Humble Monthly work? Is it a subscription?
Yeah, it's a subscription. You can sign up for 1 month, 3 months or a year I think. Basically at the beginning of the month they reveal the "premier" game in the bundle, and you can join in, and then sales are cut off for that month just before they reveal all of the other games in the bundle. And the bundle also includes exclusive little indie games.
For example, the February bundle had X-COM 2 as the featured game, and then on February 3rd they cut off sales and revealed it also included Ryse, Abzu, Steamworld Heist, Okhlos, Project Highrish, Husk, Hollow Knight preview and Oh, Deer (which is the exclusive indie game).
The March bundle has Total War: Warhammer as the featured game. You have until March 3rd to subscribe to 1 or more months, and you get access to the revealed featured game immediately.
If anyone wants to sign up for that using my referral link, that would be cool (but that wasn't my intention of bringing this up): https://www.humblebundle.com/monthly?refc=QEWlsl