C++ requires you to declare the type of every variable before using it. The type tells the compiler how much memory to allocate and what operations are allowed. The main types you will use are: int for whole numbers, double for decimals, char for single characters, bool for true/false, and string for text.
If you try to store a decimal in an int, C++ cuts off the fractional part. If you add two char values, you get a number. Type matters.