Octal (base-8) uses eight digits: 0 through 7. Each octal digit represents 3 binary bits. Octal was popular in older systems because 3 bits fit neatly into machine words.
The octal number 345 means in decimal. To convert binary to octal, group bits by threes instead of fours.
Today, octal is rare. Hex dominates because 4 bits align with bytes (8 bits = 2 hex digits). You will see octal in Unix file permissions (755, 644) and legacy code, but hex is standard everywhere else.