What does smooth the data means?

As sezleski described smoothing means convolving data with a fixed kernel of say width h.

What does it mean by convolving with fixed kernel?

Let's answer it by an example.
Assume you have 1D data (Imagine its a histogram) such as 2, 2 ,3 , 2 ,1 ,5 ,6 ,7,1 , 2 ,1
Kernel size = 3 ; Definition of Kernel= it will make average of data within its range
Now imagine this kernel is nothing but a 3 size window which we will place for all the positions of data.
Therefore, for the first position of data value2 it will do (2+2+3)/3
It will continue do that for all the positions.

This is an example of smoothing data by a kernel;