m equals the number of muffins sold, and because each muffin sells for $1.50, 1.5m represents the income made from the muffins.
Same thing with the juice boxes, j equals the number of juice boxes sold, and because each juice box sells for $1.25, 1.25j represents the income made from the juice boxes.
Add the income from both of them to get the total income, and you get 1.5m+1.25j.
