Maya numerals are how the Maya people wrote numbers. Instead of using the numbers we use (0,1,2,3,4,5,6,7,8,9) they had their own symbols to write numbers.
They used dots for 1, bars for 5 and shells for zero. So for example, if they wanted to write the number 7, they would write two dots (for 2) and one bar (for 5). So 2+5=7.
Sometimes they would put the symbols on top of each other to make bigger numbers. For example, if they wanted to write the number 20, they would put a bar (for 5) on top of four dots (for 4). So 5x4=20.
They had different symbols for different place values too. Just like how we use ones, tens, hundreds, thousands, they had symbols for ones, twenties, four hundreds, eight thousands, and so on.
It might seem a little confusing at first because they had different symbols than what we use, but once you understand how their symbols work, it's actually quite simple!