Git is software used for version control—that is, tracking the state of files. Using Git, you designate certain folders as repositories, then use commands to save the state of the repository at certain points in time. A repository is a place where things can be stored, and in this context indicates a folder that is being tracked by Git. You can perform tasks such as reverting back to previous versions of the repository, copying or sharing the repository across computers, and comparing different versions of a project.
GitHub is a cloud service for hosting Git repositories. It's something of a social network for programmers and is frequently used as a portfolio for sharing work. GitHub is used to share your code and writing with a wider community and also to find the code and writing of others. It can also be used to host websites.
Markdown is a markup language for formatting text. Unlike HTML, markdown is designed to be human-readable, which makes it much easier to write and edit.
You can think of markdown as a language read by many platforms and sites, including GitHub, to style text. Since markdown is plain text, it's also arguably more sustainable and accessible than formats like .docx. You'll notice this file is written in markdown—here is what it looks like in its raw, unrendered form.