Mujeriego directly translates to "womanizer" or "player". It's a very common term in Spanish used to describe a man who has many casual, romantic relationships.
In the song, the singer confesses that his past life on the road made him an "indolente, un mujeriego" (an insensitive person, a womanizer) who had given up on love. This single, powerful word perfectly captures the classic "reformed bad boy" story, setting the stage for how meeting this one special person changed everything for him.
This song is all about a guy who was totally over love. He says the road made him cold and he'd become an 'indolente, un mujeriego' (an insensitive guy, a womanizer). He flat out tells us, 'Dejé de creer en el amor' (I stopped believing in love). He was just going through the motions, feeling nothing.
And then, BAM! Someone new comes along and completely changes everything. He's totally captivated by her 'carita' (little face), 'boquita' (little mouth), and her attitude. He's so surprised by the feeling that he asks, '¿Qué está pasando? ... Que así de violento / Yo me enamoré' (What is happening? ... That so violently / I fell in love). He goes from being 'triste y solo, down, down' (sad and lonely, down, down) to seeing the 'más divina del Instagram' (most divine one on Instagram) and just thinking, 'wow, wow, wow'.