A baby arrives with almost no bacteria in the gut. Everything that lives there afterwards has to be collected and the collecting starts the moment they are born.
A baby born vaginally is coated on the way out. They pick up their mother's own bacteria, and those are the ones that settle in first and start the work of building a gut.
A baby born by caesarean misses that hand-over. Their first bacteria come from skin, from hands, from the theatre and the ward, a different mix, and the helpful families take longer to establish themselves. It is not that a C-section baby has no good bacteria. It is that they start with a different set, and have further to travel.
I know this one personally. My own child came that way, and this is the probiotic I gave my baby.
Two things close that gap, and both matter more than any supplement.
Skin to skin. Hold your baby against you. Your skin is not clean in the sterile sense and it is not meant to be, it is carrying the bacteria your baby is supposed to meet.
Breastmilk. It carries live bacteria, and it also carries special sugars that a baby cannot digest at all. They are not food for the baby. They are food for the baby's bacteria. Your body makes them for that reason alone.
This probiotic is the third thing, a small, gentle help while all of that is getting going. Six drops, into a spoon, into milk, or onto food.
If your baby was born early, please do not start this on your own. Premature babies are a different situation entirely, and it needs to be your doctor's decision, not mine.