Over/under 1.5 means that two or more of the selected events must happen in a match. In rugby, for example, you could wager on under 1.5 penalties. If one or less penalties happen, the bet wins. But if you bet on over 1.5, two or more penalties would have to occur.
The over 1.5 goals per game market might also be known as 2 and over. The reason for the addition of the half goal means you're not looking out for the 2 results of equal or over; you're matching 1 result where the goals are over 1.5.