It's called marginalie and it lets you write annotations in markdown files that you can then trigger to open the file side by side with the relevant part highlighted.
⁃ open with <space>m
⁃ next marginalia with ]m
⁃ previous marginalia with [m
I've been using it to annotate my explorations of software, and (as in this screenshot), to write annotations that can help people learning how to analyze code.
Download it here: https://git.sr.ht/~rbdr/marginalie.nvim/ (or wherever you get your podcasts)