It means that if you have both a
conditional, P-->Q, and its antecedent, P, then
you are entitled to claim its consequent, Q.
The authors of LPL call this rule
"conditional elimination." Many textbooks contain roughly
the same rule and call it "Modus Ponens."
(I
think that's Latin.)
Generally, any argument of the
form
- P-->Q
- Q
- Therefore, P