Cortana is the digital assistant on Microsoft devices, sort of like Siri but with a nicer voice and a name which comes straight from the Halo game series.
And like the best AI, she's constantly evolving, with new abilities coming down the pipe in Windows 10. One of them is called Commitments and it sounds like a mixture of useful and kind of creepy. Basically Cortana will sift through your email and look for key phrases which suggest you have a looming deadling - like 'I'll definitely have that report/essay/project/cake back to you by the 17th of April.' If that kind of info is flagged, Cortana will pick it out and create a reminder to let you know you've made a promise.
When the important date or time is approaching, you'll get a Cortana card to let you know all of the important information - which you presumably also remembered of your own volition. Cortana's just looking out for you, she's definitely not nagging. It's just a little nudge on from the way Google Now integrates your communications into calendar appointments, and a step down the line of letting our phones handle even more of our daily lives.
Commitments is coming to the Windows 10 Insider program at the moment, so there's a good chance it will roll out in a future update to all users.