I posted on here when HashiCorp announced that they would switch to the BUSL license for all their software, which would result in none of their software being open source anymore. The most significant of that software was Terraform. The reaction by the Open Source Community was completely negative, and a group calling themselves the OpenTF first posted a petition asked HashiCorp to reconsider, then decided to fork Terraform and search for a foundation to support them.
The Linux Foundation picked up OpenTF, and have renamed it OpenTofu. I will personally be switching to OpenTofu for all of my future (and possibly some of my previous) projects.
As Abhishek said, its an infrastructure as code tool that can be used on basically every public cloud (the big ones being AWS, Microsoft Azure and Google Cloud Platform). There are several other tools like this, but Terraform is by far the one that is most used in industry.
My understanding is OpenTofu will actually have more full time devs committed to it than Terraform does now. Not to mention that there will be people contributing to it even if its not their job, unlike Terraform after their annoucement. I am even thinking of contributing.
I recommend you do. OpenTofu says that they will be a drop in replacement for Terraform versions 1.5.x. As time goes on, that may not happen though.