I, in roman numerals, means 1 and for each additional I you add 1 to it. When you reach 5 the letter representing the number is V. If you add an I before it you subtract one so IV equals 4. If you add an I after it, you would add one so VI is equal to 6. You can find more information here: http://www.novaroma.org/via_romana/numbers.html
