Any introductory C book will discuss pointers. If it doesn't, it's not worth the paper it's printed on.
A "pointer" is a variable that stores the address of something in memory. You specify exactly what
it points to by giving the data type, and then specify it's a pointer by using a *. In this case, a "char *" means that the value is the address in memory of a "char" variable. A char is a type of variable typically used to represent a letter. In C, a "string" is a collection of char values. Each char in the string is stored in consecutive memory locations. The end of the string issignified when the special character (NULL) is encountered. NULL has a value of exactly 0. Thus, in memory, the string "pointers_are_fun" would look like:
p o i n t e r s _ a r e _ f u n 0
To use the string in the program (for printing, analysis, or whatever), you must refer to it by the starting address of the string. That is what a "char *" is if you recall: a memory location containing one or more characters.
So if you expand on that idea, a "char **" is the location in memory, of one or more strings. Think of it like the "pointers_are_fun" example earlier. Except, instead of letters, you have "char *" values stored in sequence. In other words, you have a collection of addresses that each point to a string of characters. You have to make two "hops" to get to a given string: Starting at "char **", pick out one of string you want to examine, then use that value as a string. It's a little hard to describe. Perhaps someone else will add a little more to help clarify it.
Like I said, any introductory C text ought to explain this in much greater detail. Pointers and strings are fundamental parts of C programming. To leave them out would be criminal.