Short vocal interjections between main lines or at the end of phrases — 'yeah,' 'oh,' 'come on,' or short melodic ornaments.
Listen to the beat and find gaps between phrases — that's where ad-libs go. They shouldn't compete with the main line.
Ad-libs add texture, energy, and 'liveliness.' They make a recording less sterile and more 'human.'
'Yeah!' - Usher, 'Oh-oh-oh' - Beyoncé, hip-hop ad-libs at the end of every other line.