A single memory is never about just one thing.
A photograph from a summer at the lake is about that summer — but it's also about the people who were there, the house you stayed in, and your father, who taught you to swim off the end of the dock. All of it, at once. One memory, with several strands of meaning running through it.
A thread is one of those meanings, made into something you can follow.
When you make a thread — when you say this is about my father, or this one is about the summers at the lake — you aren't filing anything away. You're remembering. You're deciding what a memory means, and where it belongs. You could add a single photograph and one thread, and nothing else — no date, no place, no names — and you'd already have done something that matters. You'd have said what it means.
And a memory can belong to as many threads as it truly belongs to, because that's how remembering works. Nothing in your life filed itself under one heading. The lake photograph is about your father, and that summer, and the cousins who came every year — and it can sit on all of those threads at once.
This is what makes a thread different from a collection. A collection is a place where memories live: you put them there, and that's where they sit. A thread isn't a place. It's a meaning that runs through your memories, wherever they happen to live. The same photograph can rest in one collection and still carry a dozen threads.
And once a thread runs through your memories, you can follow it. Pull the thread for your father, and every memory he's part of comes with it — no matter which collection holds it, no matter who added it. The story he told at the table, the photo from the dock, the letter in his handwriting. Gathered, because they share a thread.
You don't have to plan any of this, or know the shape of things before you begin. You can say what a memory is about the moment you add it, and the threads will gather meaning on their own, as you remember more.
A collection is where a memory lives. A thread is what it means to you.
For the unit a thread runs through, see What Is a Memory? For the personal space where your threads can live alongside everything else you keep, see Your Library.