Humble Monthly is as straightforward as it’s ever going to get. You pay a monthly fee of $12 (or the currency chosen for your region) to get keys for 7+ games every month. Typically there are one to three “Early unlocks” provided as an incentive to pay for the whole bundle, while the hidden games are revealed once the time to purchase/subscribe runs out. The keys can typically be redeemed on Steam, rather than GOG.COM or other platforms, and they are yours to keep whether you stay subscribed or not.
In addition to the games active subscribers receive a 10% discount on games in the Humble Store, as well as access to Trove: a curated collection of DRM-free games you can download from Humble and can keep on your HDD/SSD as installation files. Trove has titles such as Torchlight, The Sexy Brutale, or Getting Over It with Bennet Foddy, among many others.