Run ls -l to see permissions. The first column shows a 10-character string such as -rwxr-xr--. The first character is the file type (- for regular file, d for directory). The next nine characters are three sets of rwx (read/write/execute) for owner, group, and others respectively.

ls -l /usr/bin/python3

Change permissions with chmod. You can use symbolic or octal notation:

chmod u+x script.sh        # add execute for owner
chmod go-w sensitive.txt   # remove write from group and others
chmod 644 public.html      # rw-r--r--  (octal)
chmod 755 myapp            # rwxr-xr-x  (octal)

Change ownership with chown (requires sudo):

sudo chown alice:www-data /var/www/html/index.php
sudo chown -R alice:alice /home/alice/projects/

A common pattern for web files is 644 for files and 755 for directories. Keep sensitive config files (passwords, private keys) at 600 so only the owner can read them.