3 linked guides. Start at part 1 or open the chapter that matches your question.
When to use chmod 755 versus 644—what each octal mode grants, why directories need the execute bit and how to pick safe defaults for files and folders.
How to use chmod -R to change permissions on a whole directory tree without making files executable—plus the find and capital-X tricks that keep folders usable.
The two ways to write chmod—octal numbers like 755 and symbolic forms like u+x or go-w. How each works, when to prefer one and how they map to the same bits.