The term "deep house" has been used with (at least) two distinct meanings:
Originally, the term was roughly synonymous with "soulful house". Deep house, in this sense, is house music influenced by jazz, soul, and gospel. Vocals (both male and female) are common, and lyrical themes include positivity, community, love, self-belief, and overcoming hardship. Overly synthetic sounds are downplayed, in favor of pianos, organs, horns, bass guitar, etc. Tracks sometimes follow more of a conventional verse-chorus-verse structure than other forms of house.
In the 2010s, "deep house" has been applied to a newer style of house that tends toward the dreamy, laid-back, subdued, atmospheric, reflective, and minimalistic. (This sense of the term is roughly synonymous with "lo-fi house".)