What exactly is a "deep cleaning" and does everyone at some point have to have this done?

Yes: A deep cleaning is just one where the dentist or hygienist uses an instrument called a curette or scaler. It is as only as deep as the pocket of gum (gingiva) above the bone. Healthy teeth will have shallow pockets and the "deep" cleaning is not a big deal. If there is significant pocketing, then it is indeed deep to get the depth of the pocket clean.

Gum Disease: Deep cleaning is a procedure called scaling and root planing that is used to treat periodontal (gum) disease. It is meant to clean tartar and bacteria from below the gumline. A traditional cleaning focuses above the gumline. Deep cleaning is needed when there is evidence of destruction of supporting tooth structures such as gum and bone. Some people will go through life never needing it.

Extensive cleaning: A "deep cleaning" is part of the treatment for periodontitis or bone disease. It is the removal of tartar & bacterial toxins from the root surface. Only individuals who develop periodontitis need a "deep cleaning". This is treatment of an infection. Many today are also including laser therapy. The laser kills bacteria & aids tissue regeneration. Improved home care is essential for success.