Mojang have decided that as of next year all of their games will require a Microsoft account to play, including Minecraft Java Edition. Don't panic just yet, because while this does mean that you'll need to log in with Microsoft to play the Java edition, it's not being merged with the Windows 10 (or Bedrock) version of Minecraft. You'll still be able to mod your game and use Java servers. Mojang simply say that one of the big reasons for the move is increasing account security.
So, early next year, Mojang will start instructing players on how to migrate their accounts from Mojang to Microsoft. They emphasise in their post that this is mandatory, and if you don't make the transition, you won't be able to log in and play at all.
It sounds kinda harsh, but it does seem to be for good reasons. Mojang immediately bust out the positives in reassurance, noting that Microsoft accounts will allow you to enable two-factor authentication, have access to better parental controls, and use chat and invitation blocking.
The account migration will mean all your Minecraft PC games (so Dungeons and Bedrock, if you own them) will be linked under the same account, but importantly, they won't be merging. Java will still be its own thing: you'll still have your in-game Java username, mods, skins and the ability to play with your Java pals.
Any new players trying to sign up for Java will make Microsoft accounts rather than Mojang accounts starting this Autumn (so soon, if not now). Existing players will receive emails and help from Mojang at the beginning of 2021 to start making the switch. When you're done, you'll get given a cape for your troubles, too (to clarify: an in-game cape, though I think a real-life one would be much more fun).
If you're after more info on the account transitions, there's already an FAQ that addresses a lot of the worries players might have. It doesn't sound as drastic as it seems at first glance, and if you're still unsure about things, Mojang have also made that weird cutesy video explainer above to go along with it.