Data validation limits what can be typed into cells. The quickest way in is Insert > Dropdown; Data > Data validation opens the same Data validation rules panel on the right.
Each rule has an Apply to range and a Criteria:
- Dropdown: you type the options into the panel, one per box, and can give each a color. The cells show them as chips.
- Dropdown (from a range): the options come from cells, often on a separate tab such as
Team!A2:A7. Add a name there and it appears in every dropdown.
- Is between, Greater than and other number tests check typed numbers.
- Checkbox turns the cells into tick boxes that hold TRUE or FALSE.
Under Advanced options, "If the data is invalid" decides what happens to a typed value that breaks the rule. Show a warning accepts it but marks the cell with a red corner. Reject the input refuses it, which is what you want for a field others fill in.
Press Done to save the rule.