When light is incident on a barrier with a narrow slit, we observe an interference patten due to the non zero width of the slit. Every point on the wavefront behaves like a light source and we observe an interference patten due to the interference of light from opposite sides of the slit.

This is the top pattern in the diagram.

When light is incident on a barrier with two parallel narrow slits, each slit behaves almost like a point source of light.

This means that each slit gives rise to almost circular wavefronts that spread out from the slit. We observe a diffraction pattern like the top pattern in the diagram.

The overall pattern produced is found by taking the single slit diffraction pattern as the envelope for the double slit diffraction pattern, because in effect, the single slit is split into two narrow slits.