Why computer understands binary language only?

All forms of data within a computer are based on digital circuits. A digital circuit can have exactly two states, one of "on" or "off". They are not able to represent intermediate values, such as "half-on" or "three-quarters off"; this would be known as an analog circuit. Since there are only two possible states for any single datum (called a "bit"), computers can only understand series of these bits ("bytes", "kilobytes", etc).