Is H2S dihydrogen monosulfide or dihydrogen sulfide, please explain?

The quite official name would be dihydrogen monosulfide , though there is a trend in IUPAC to the name sulfane .

The first name exactly describes what the molecule is built up from: 2 (di) hydrogen atoms and 1 (mono) sulphur atom.

The other one, with the -ane ending describes an atom (in this case sulphur) that has its bonds 'saturated' with hydrogen (compare methane). These new names are not universally accepted yet. Water would be called oxane, and ammonia azane (az- is often used for nitrogen).

But: Chemists are lazy, and if the composition is clear and unambiguous, they often leave out the 'counting'. Then it would be hydrogen sulfide .

(this is not the case with the carbon oxides: there are two of them, so are they explicitely named carbon monoxide and carbon dioxide)