Three digits: owner, group, everyone else. Each digit adds up read (4), write (2) and execute (1).
| Digit | Means |
|---|---|
| 7 | read, write, execute |
| 6 | read, write |
| 5 | read, execute |
| 4 | read only |
| 0 | nothing |
So 755 is "I can do everything, everyone else can read and run it": the usual for directories and scripts. 644 is the same without execute: the usual for files. 600 is for things like private keys that nobody else should even read.
chmod 755 deploy.sh
chmod 644 index.html
chmod 600 ~/.ssh/id_ed25519Directories need execute to be entered, which is why chmod -R 644 on a folder locks you out of it. Use chmod -R u+rwX,go+rX instead; the capital X only adds execute to directories and to files that already have it.