What does the permissions 644 mean for a file?

File permissions 644 in Unix-like systems mean:

• The file owner (user) has read and write permissions (6 = 4 + 2).
• The group (group) has only read permission (4).
• Other users (others) also have only read permission (4).

In binary form:

• 6 = 110 (read + write)
• 4 = 100 (read only)
• 4 = 100 (read only)

Thus, the owner can modify the file, while everyone else can only view its contents.