cat reads a file. echo plus an arrow writes one.

You can make empty files. Now: putting something in them, and seeing what's already there, without opening an editor.

cat

cat prints the contents of a file straight into the terminal:

cat notes.txt

If notes.txt says "buy milk", the terminal prints buy milk and hands you a fresh prompt. cat doesn't open the file in anything and doesn't change it. It just dumps what's inside onto the screen so you can read it fast. (The name is short for "concatenate", which is a second thing it does that you can ignore for now.)

cat is most useful for a quick look. Later in the course you'll cat a file to check that a command actually wrote what you expected.

echo with an arrow

You met echo in lesson one: it prints its argument back to you. echo buy milk just prints buy milk to the screen.

Add a > and a filename, and instead of printing to the screen, it writes into the file:

echo buy milk > notes.txt

Nothing prints. The text went into notes.txt. Run cat notes.txt and you'll see buy milk come back. The > is the redirect: it takes output that would have gone to the screen and sends it into a file instead.

One sharp edge worth knowing now: a single > replaces the file's whole contents. If notes.txt already had ten lines, echo something > notes.txt throws all ten away and leaves one. There's a two-arrow version, >>, that adds to the end instead of replacing. You don't need >> yet. Just know that one arrow overwrites, so you don't lose work by surprise.

Try it: echo first note > log.txt, then cat log.txt to read it back.