Do not attempt to adjust your set. We are controlling transmission.
changes who can read and write your files (!!) which is probably something you want to keep control over. the basics:
works in octal (as opposed to decimal, binary, hex, etc. it's base 8). 4 means read, 2 means write, and 1 means execute. add together the digits for the behavior you want to get the number you need.
takes an octal number as its first arguments, as in 755 or 600. each digit is computed via the method described above, then each position means something different. the first digit is you and you alone. the second digit is the group the file belongs to (but that’s another post). the third digit is everyone else. mind you,
can read everything and ignores all this permissions stuff.
chmod 600 ./path/to/myfile.txt
flag right after
to make it recursive, to act on a directory and all its files.