If you have notes to yourself scattered throughout your hard drive, you might need a notes application to collect and organize your personal reminders. A notes system can help you track ideas, important tasks, and works in progress. A popular application that isn't open source is Notion, but here are two options that respect your privacy and data.
Standard Notes is an open source (AGPL 3.0 license) notes application featuring a password manager, a to-do list, and, of course, a great system for writing and storing notes.
One of the most important things about taking notes is finding them again, so organization is critical. Standard Notes uses an intuitive and natural tagging system to help you organize your content. You assign hashtags to each note to classify it.
Standard Notes is extensible through plug-ins. There are plug-ins for LaTeX, Markdown, code snippets, spreadsheets, and more. There's even an option to publish to a blogging platform, should you want to make some of your notes public.