The Humble Monthly Bundle is a "curated bundle of games," valued at over $100 at retail, given to subscribers for $12 per month—or less if you sign up for longer-term subscription plan. You can cancel at any time (once your subscription period is over, obviously), but you'll keep whatever games you pick up while you're a member.
Long-term subscriptions are a bit of a roll of the dice, but the bottom line is that, on a month-by-month basis, it's a good way to get games on the cheap. Such as in the current bundle, which went live today with the Paradox space strategy game Stellaris on the marquee. Stellaris normally sells for $40/£35/€40 and the lowest it's ever gone is $20, so 12 bucks is a pretty sweet deal if grand strategy in the cold void of space is your thing.
The Humble Monthly Bundles include other games as well—it wouldn't be much of a "bundle" if it was just one—but they like to keep the bonus stuff under wraps until the package has expired. Last month's bundle, for instance, included This Is the Police, Undertale, Metrico+, The Turing Test, Gonner – Press Jump to Die Edition, Kevin Eastman's Underwhere issue 1 (the first Humble Monthly comic), Super Rude Bear Resurrection, and the "Humble Original" A2Be – A Science Fiction Narrative, all of them on top of the Dirt Rally and Inside headliners. You get ten percent off all Humble Store purchases while you're a subscriber, too.
The Humble Monthly Stellaris Bundle is live now and will remain so until midday on June 2.