Can someone explain me Humble Monthly Bundle please?

You get the monthly bundle, which you won't get any details about until it's too late (when everyone receives it). Basically, it's a bundle pre-order. You get what's in the bundle, like everyone else, but only if you subscribed before it was live. They also sometimes give some instant keys for subscribing to the monthly, in which case they tell you what game you'll receive for this promo.

For reference, here are the past Monthlies: https://en.wikipedia.org/wiki/List_of_Humble_Bundles#Humble_Monthly_Bundles

You can see their past Monthly Bundles here:

https://en.wikipedia.org/wiki/List_of_Humble_Bundles#Humble_Monthly_Bundles [updated with the better link from jbondguy007]

Imo the last one was really worth it, it was my first Monthly as well. Got it mainly because The Witness on itself (as the game they reveal first) was already worth it for the price. Plus, got Black Mesa, Kingdom - New Lands, event[0], Layers of Fear (enjoyed getting all of those) and plus a couple of other nice games. Only Tumblestone wasn't my cup of tea to be honest.

But yeah, I've seen case of "bad" bundles. They were not exactly made of shit games, but more niche games I would say. So there will always be someone who will enjoy it, others who will hate it, some bundles will be more popular, some will only praise a few, and so on...

It all depends if you like the main game enough and consider everything else to be profit, in my opinion. Tho you can always trade the others, make giveaways for it, so its not like you NEED to activate them.

You'll hardly lose anything by buying a Monthly with a main game you know you'll enjoy, unless a lot of games from the bundle comes out in standard $1/BTA/$15 bundles a couple of months later, which already happened more than I would say its cool. So in that sense its also a kind of gamble.