The second number (the share) is the percentage of households watching. The third and fourth numbers tell how the episode fared against its competition on the three other networks and against all of the other programs shown for the week.
(The four major U.S. networks typically show a total of about 70-80 programs each week.)
So, in this case, the series finale of "MacGyver" drew about 13.1 million viewers. 26 percent of the television sets on at the time were tuned to the show, which won its time slot and placed 15th for the week.