Unlike vomiting - when a baby's muscles forcefully contract - reflux is a baby effortlessly spitting up the milk they've just eaten.
Dr Keir Shiels explained that when babies drink milk, it passes down into the stomach. Between the food pipe and stomach is a valve, which for adults is very tight.
However some babies have floppier valves, which causes reflux.
"If your baby is crying a lot when laying flat or vomiting a lot when feeding your doctor may suggest your baby has got reflux," he explained.
"Of course all babies can cry when lying down, but if your baby is particularly uncomfortable when flat and comfortable when sat up, reflux is the problem."
Dr Shiels went through three things parents can do to soothe the symptoms babies may be experiencing as a result of reflux.
Watch the video above to find out how.
'Baby 101' is HuffPost UK's parenting video series with paediatrician, Dr Keir Shiels, addressing topics mums and dads may face in their first year of being a parent. From colic to trapped wind, crying babies and their nappy contents, he answers some of the most common questions new parents have.