any word beginning with an upper-case letter is a Prolog variable

This is the way logical conjunction is expressed in Prolog (that is, the comma means and )

the semicolon ; is the Prolog symbol for or

it is usual to use a suffix / followed by a number to indicate the predicate’s arity

the :- means implication, the , means conjunction, and the ; means disjunction

This will introduce us to the three basic constructs in Prolog: facts, rules, and queries.