In Layman language:
The normal programs run in user space and kernel program runs in kernel space. Now what is the difference between these spaces? Kernel space contents makes the guy using it much more powerful than the guy who could use the user space. What are the contents is not the concern for now. There is whole bunch of theory and practicals behind it. (And also i don't remember the whole of it for now).
When ever a guy(normal program ) using the user space does something he is does one of two types:-
First a work which which is totally confined in his locality and is not affecting others
Second the vice-versa. This it cannot do on it's own since it does not know how to do it (say it twice) since the space he is using (user space) does not have information( the instructions or the set of instructions) which tells it how to do it . This is obviously for security.
Then how it should be done. Here comes the kernel space.
Since only kernel is the guy who could use the kernel space the control is transferred it. Now kernel using the kernel space which contains sufficient information does it on behalf of normal program and returns the status of work performed.
Hope this would help in layman terms. Apologies for the typos.