Explain how bright fringes arise in Young's double slit experiment

To answer this question, we need to think about 3 things: interference, phase and path difference

In the experiment, we have monochromatic light (from a laser) going through two slits and then bright and dark fringes appearing on the screen. (I will draw the experiment on the white board) 

What the monochromatic light means is that the light shining through the two slits act as two coherent sources of light (at the same frequency). In order for bright fringes to occur, the waves of light need to interfer constructively. This means that the displacement of the waves when they intefer with each other add together - they superimpose to make a larger amplitude ie the bright fringe. To interfer constructively like this, the waves must arrive together in phase and have a path difference which is a whole number of wavelengths (1 wavelength, 2 wavelengths etc)