The numbers stand for this:
- 0 = no permissions
- 1 = execute
- 2 = write
- 3 = write and execute
- 4 = read
- 5 = read and execute
- 6 = read and write
- 7 = read, write, and execute
These permissions are set on a Owner - Group - Other (everyone) basis. Therefore....
755 means the owner can read, write, and execute. The owning group and everyone else can read and execute.
The ownership part comes in when you either FTP or ssh into your account to upload/edit files. If your hosting account was setup with 444 or 555, you wouldn't be able to upload or edit files.
The reason that the Owner and Group settings are not necessarily the same is that an Admin may combine several users into one group (take "ftpusers" as an example). By specifying the permissions to set files and folders as writable ONLY by the Owner, no other group member can make changes.
A good rule of thumb: Give group the same permissions as others (everyone else). ONLY give write permission to the Owner.