wiring for an oldish house
Both the black and red are likely ungrounded (hot) conductors. However, one or both of them are likely switched.   That is, one or both of them are energized only when a switch is closed.  

Test the wires with your meter, and determine which one is controlled by the switch. Once you figure that out, connect only that wire to the black wire on the fixture.  Cap the other wire off, and tuck it safely away inside the box.