A function argument is a specific input to a function. For example, the VLOOKUP function takes four arguments as follows:

```
=VLOOKUP (value, table, col_index, [range_lookup])
```

Note most arguments are required, but some are optional. In Excel, optional arguments are denoted with square brackets. For example, the fourth argument in VLOOKUP function, range_lookup, is optional and appears in square brackets as shown above.

Finally, many Excel functions accept multiple optional arguments, which are denoted with an ellipses (...)

For example, the COUNTIFS function accepts multiple and optional range and criteria pairs, which can be represented like this:

```
=COUNTIFS(range1,criteria1,[range2,criteria2],...)
```

This means you can optionally add additional arguments in pairs: range3/criteria3, range4/criteria4, etc.