When we collect data, we can count objects using a 5-bar gate. We count one object, and draw one tally line straight down. Then we count another object, and draw another tally line. When we have four tally lines, we draw a diagonal line through them. We now know we have five objects. Then we start again with another 'gate'.
This makes it easier to count larger numbers. Remember: one tally line = one object.
This shows 5 + 5 + 5 + 3 = 18
You might find it easier to count by saying: 5, 10, 15 plus 3 equals 18.