The expression that tells how the numbers in a determined set of cells are to be calculated is called a formula.

Here's a breakdown:

* Formula: A formula is a combination of cell references, operators, and functions that tell the spreadsheet program how to calculate a result.

* Cell references: These identify the specific cells containing the numbers you want to use in the calculation. For example, A1, B2, C3.

* Operators: These are symbols that indicate the type of mathematical operation you want to perform. Common operators include:

* `+` (addition)

* `-` (subtraction)

* `*` (multiplication)

* `/` (division)

* `^` (exponentiation)

* Functions: These are pre-built commands that perform specific calculations, such as SUM, AVERAGE, MAX, MIN, etc.

Example:

Let's say you want to calculate the total cost of items in cells A1, A2, and A3. The formula would look like this:

```

=SUM(A1:A3)

```

This formula uses the SUM function to add the values in cells A1, A2, and A3, and the `=` sign indicates that it is a formula.

In summary:

* A formula is a combination of cell references, operators, and functions that tells the spreadsheet how to calculate a result.

* Formulas are essential for automating calculations and making your spreadsheet more efficient.