Labial consonant refers to a consonant that is produced with the lips. “labial” comes from Latin: Labia – the lip (lt.). In the case of the given devanagari consonants these sounds are all produced by pressing the libs together and then suddenly releasing/opening them. The sound is produced in the moment of release.
प फ ब भ – pa pha ba bha
They follow the same system of order as the gutural, palatal, retroflex and dental consonants:
प – pa : Non-sounding, non-aspirated
फ – pha : Non-sounding, aspirated
ब – ba : Sounding, non-aspirated
भ – bha : Sounding, aspirated
Watch out for the difference in pronunciation between the two aspirated consonants फ and भ.
Also be sure that you do not just pronounce फ pha as a F only. The P is actually still audible. That is also a reason why unpractised Nepali native speakers have difficulties pronouncing the English F. And in combination with a difficulty to pronounce the English SH a native Nepali speaker might end up saying something that sounds more like “piss” when they actually want to say “fish”.