When something summates, it adds up or combines to make a total.
When things summate, it usually means they are coming together to make something bigger or more complete.
Examples:
The numbers summate to 100.
The colors summate to make a beautiful picture.
She summates the money to see how much she has.
The ingredients summate to make a delicious cake.
All the little drops of water summate to make a big ocean.
The pieces of the puzzle summate to reveal the whole picture.