Use a browser-based chmod calculator: tick the read, write and execute boxes for owner, group and others and it gives you the octal value like 755, along with the matching symbolic string. It runs on your device and takes the guesswork out of Unix file permissions.
Unix permissions have three groups, owner, group and others, each with read, write and execute bits worth 4, 2 and 1. You add the bits within a group to get one digit, then read the three digits together. Read plus write plus execute is 4 plus 2 plus 1 equals 7, and read plus execute is 4 plus 1 equals 5, which is how rwxr-xr-x becomes 755. A calculator does this addition for you and shows both notations.
755 is typical for directories and executables, where the owner can do everything and everyone else can read and traverse. 644 suits regular files, giving the owner read and write and others read only. 700 locks a file to its owner entirely. Ticking boxes instead of memorizing these avoids the classic mistakes, like accidentally making a file world-writable.
Owner has read, write and execute (7); group and others have read and execute (5 each). It is common for directories and executables.
644 gives no execute bit, suiting regular files; 755 adds execute, which directories and programs need.
No. The calculation runs entirely in your browser.