OpenTofu is an open-source infrastructure-as-code tool. Teams describe cloud and on-premises infrastructure in configuration files, then plan and apply changes from the command line. The project describes itself as a drop-in replacement for Terraform and was created as a fork after HashiCorp changed the Terraform licence. It is distributed as a command-line program that organisations run on their own machines or in their own pipelines.
OpenTofu was started by Gruntwork, Spacelift, Harness, env0, Scalr and others. The project page states that it is under the stewardship of the Linux Foundation, as a series of LF Projects, LLC, and the repository is published under the Mozilla Public License 2.0. The Linux Foundation is a US non-profit organisation, so the project's governance falls under US law. For a European organisation this means the software is open source and can be operated by the organisation itself. The platforms that store state or run plans are a separate choice, which should be assessed on their own terms.