If an attribute on your records can have more than one value, it is known as multi-select. For example, an attribute "Flavor" on wine records can have values "Apple" and "Apricot".
The schema for your records controls whether a standard attribute can have a single value, or multiple values. Attributes with multiple values can be of two types — multi-select AND (also referred to as multi-and), and multi-select OR (also referred to as multi-or) attributes. Respectively, refinements on such attributes are also known as multi-select refinements.