Multiple choice fields are field types that allow users to make a multiple selections from a defined list of choice options. This options can either be populated inline with the choice field options or leverage a pre-defined choice list. The choice options can have both a label and a value.
When the option 'Allow "other"' is enable, the users capturing the data can manually enter in value that they wish to enter for this field. The manually entered in value is not added to the choice list for future selections.
When you are making selections on the web app. You will either need to shift-click or control-click to make multiple selections.
Multiple choice fields can either have the choice options defined in the field's setting popup (pictured below) or use a predefined choice list.