We use chmod command to change the permissions of a file in Unix. In this command we can pass the file permissions in the form of a three-digit number.
In this number 755, first digit 7 is the permissions given to owner, second digit 5 is the permissions of group and third digit 5 is the permissions of all others.
Also the numbers 7 and 5 are made from following rules:
4 = read permission
2 = write permission
1 = execute permission
So 7 = 4 + 2 + 1 = Read + Write + Execute permission
5 = 4 + 1 = Read + Execute permission
In out example 755 means, owner has read, write and execute permissions. Group and others have read and execute permissions.