Apples with Apples.
Simplifying expressions by adding or subtracting terms that have the exact same variable part.
The key step to solving multi-step equations.
Look at the 'Last Name' (variable). Only families can hang out together.
In Plain English: You can't add '3 chairs' and '2 tables' to get '5 chables'. They are different. In math, you can only combine x's with x's, and regular numbers with regular numbers.
In The Real World: Inventory lists. Combining counts from two different warehouses.
Combining x and x². They look similar, but x is a line and x² is a square. They cannot be added.