Convolving a histogram with a kernel

Question

I have some raw histogram driven out of a discrete dataset. In order to make it smooth, I have been told to convolve it with a kernel (ex. a Gaussian or a Hamming one) which values sum up to one. Now the question is,what actually is a kernel ?And what should sum up to one and why?Is this a simple filtering using a Gaussian function?How should I implement this in MATLAB? Thanks in advance.