The ampersand character (&) is a special operator in Excel that is used for concatenation. Concatenation is just a fancy word for "join". In the formula, the text in B5 "Susan" is being joined to a space character (" ") and then to the text in cell C5, "Chang".
When you use concatenation in a formula, be sure to enclose literal text in double quotes (""). Do not enclose the ampersand (&) in quotes, unless you want it to appear in the final result.
You can also use the CONCATENATE function to join text like this:
The CONCATENATE function keeps things tidy, but the ampersand creates shorter, more flexible formulas.