There are 24 hours in one day, but the day can be measured by splitting it into two halves. The first 12 hours of the day - from midnight to midday - are called AM, and the next twelve hours are called PM.
Roman numerals
The Romans used a different numbering system from the one that we use today. Their numbers were shown by letters.
I = 1, V = 5, X = 10, L = 50 and C = 100
Here are some examples
V is 5, so VI is 5 + 1 or 6.
IV is 4. When the I is before the V it means 1 less than 5.