Imagine you are on a big bus called the Clapham Omnibus. There are many people on this bus and they are all very different from each other. Some are old, some are young, some are rich, some are poor, some are happy, some are sad.
Now, let's pretend that one of these people, who is not special or different in any particular way, is called the "man on the Clapham Omnibus." This person is just like any other regular person on the bus.
The reason we talk about the man on the Clapham Omnibus is that he represents what we call an "ordinary person." In other words, this is what we imagine a regular person would think and do in a particular situation.
Sometimes in court cases, lawyers will argue that something is reasonable or not reasonable based on what the man on the Clapham Omnibus would do or think. This is just a way of saying that they are trying to figure out what most people would do or think in a particular situation.
So basically, the man on the Clapham Omnibus is just a made-up character that helps us understand what most regular people might do or think in a given situation.