Could you explain what a lighter aboard a ship barge is?

[Brian2722]

Recently, while I was bored, I came across the term "lash barge" while watching something on YouTube about transporting cargo on rivers in the US. It seems to refer to barges that are transported on large ships. Does anyone here understand how it works and why such barges are useful for small ports? I'm curious about how it saves time and money. There were a lot of comments under the video, but I was too lazy to read them.