A fact is a Prolog clause with a head with no variables in it, and no body, like

likes(mary, pizza).

as opposed to this rule:

happy(Person) :-

Bodiless clauses with variables, like …

member(Item, [Item | RestOfList]).

… behave like rules in that they provide a general way of knowing that, for any Item, that item is a member of a list of which it is the first item (see also member). The clause above is likely to generate a SWI Prolog warning message like this:

        Singleton variables: [RestOfList]
to tell you that the variable RestOfList is only mentioned once in the code (so may be a spelling mistake, for example). You can suppress the warning by writing, instead:
member(Item, [Item | _]).
using a don't-care variable, although this may be more difficult for a human to follow, particularly a beginnner at Prolog.