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Click permission checkboxes and get the numeric mode, or type 755 and see exactly which bits that sets. Owner, group, and other are laid out as a grid so the mapping is visible rather than memorized.
Convert between numeric and symbolic Linux file permissions. Click checkboxes or type a number to calculate both representations.
Numeric
Symbolic
-rwxr-xr-x
Command
chmod 755 <file>
The numbers are a base-8 encoding. Read is 4, write is 2, execute is 1, summed per role. So 755 is owner rwx (4+2+1), group r-x (4+1), other r-x. Once you see the grid the arithmetic stops being a memory exercise.
Two modes are ones to memorize, because getting them wrong has consequences. 600 is what SSH demands for a private key. Anything more permissive and ssh refuses to use it. 777 is almost never the right answer; if a permission problem only goes away at 777, the real issue is ownership, and chown is the fix.