A waiter had 4 customers. After some more arrived he had 10 customers. How many new customers arrived?
The waiter had 4 customers. 
Then, some more arrived. 
That means that the number of customers increased by some amount. 
Now the waiter has 10 customers. 

We can write this as an equation: 

4 + some number = 10

Some number equals 6. So, 6 new customers arrived.