What Does 'Not Fulfilling Your Spouse' Truly Mean?

Assalamu alaikum, I've come across many accounts of husbands struggling to fulfill their wives, or wives having difficulty fulfilling their husbands. What do folks actually mean by that? Is it a physical issue, an emotional one, linked to how we communicate, or something totally different? I'd love to hear various perspectives and personal experiences in the light of our deen.