Multiselect works the same as the single select field. You can select options from the predefined list, except that you can choose more than one option in a record.

How does it work? 

1. Choose Multi Select from the list of fields available.
2. This opens up a dialog box. Click the + ADD AN OPTION button to add the list of options/choices.
3. Using the delete icon, you can also delete the options that are not required.
4. Use Sort (A-Z) / (Z-A) option to arrange the options at your convenience to pick one easily when you fill in the records.
5. Colored Option is to differentiate between the choices quickly. By default, choices are color-coded, but you can customize them.
6. Using the drag & drop option, You can swap the choices between each other.
7. Hit Enter.

Now, when you click on the + icon in the record, you can add more than one option to your record.