Why would we see brightening of stars due to lensing?
Gravity bends light, and light passing by an object with mass is curved in the direction of that mass (see Figure 22.11 of your text). If a massive object like a MACHO moves across our line of sight to a star, the light from the star passing on either side of the MACHO- which in the absence of a MACHO would just keep going and miss us- instead gets curved towards us. In effect the MACHO acts like a focusing lens (its size is small enough that it itself doesn’t block much light). Since more starlight reaches us when the MACHO is in the line of sight, the star brightens. Here’s a link with an animation which may help you see how this works.
