If I were to explain in terms of Reverb and Delay,I would say that you imagine a room.And the more wet you make a sound in terms of Reverb and Delay,the more reflections will that sound have in that room.
You might want to consider Reverbs and Delays as a last resort though,because these effects can effect their space in the mix.
Using Compression and adjusting Fader levels can give you a Deeper Mix.
I think it's mentioned above that you should cut high frequencies to make it deep.The explanation for that being that lower frequencies have more energy,and hence travel longer distances as compared to higher frequencies.In simple words,cutting high frequencies can provide the sound more energy for it to be audible at deeper levels of the Mix.
Hope this helps.