This is a really vague question, so it'll be difficult for anyone to give you a proper answer.

Whenever you have lateral forces that need to be carried by shear walls, braced frames, etc., those forces have to get to those supporting elements somehow, commonly through a diaphragm. So if you have lateral force (seismic, wind) accelerating/applying force on your floor, you will need to design it as a diaphragm to ensure it can carry the load to your walls/frames properly.

Can you be more specific about what you mean?

This comes down to understanding both vertical and lateral load paths.
If you've been doing some structural work, you probably are somewhat comfortable with vertical load paths.
Diaphragms are a key element of the lateral load path for buildings.
Imagine wind pressure pushing on your wall. You design the wall as being supported at the ground, and at the roof say to resist the wind loads.
Well the element that provides the support at the roof is the diaphragm. The diaphragm takes those loads and moves them over to the lateral resisting elements.

Slabs on grade can also be considered a diaphragm where they are used to brace retaining walls or transfer lateral loads from above.