A diffuser is a semitranslucent material placed between a light source and a subject which scatters light beams to soften light. A bare strobe produces a hard light, but if soft light is desired the strobe's light can be diffused. An example of a diffuser is a softbox which is placed around a strobe with diffusion material on its front face.
The sun is a hard light source which is often diffused with a scrim. By placing a scrim between the sun and the subject, the light rays are scattered and harsh sunlight becomes softer.
It's worth noting that diffusing light will reduce its intensity and camera settings will need to be adjusted to compensate.