sparse matrix using linked list C++

[HarryPho]

I'm coding the program that using linked list to store a sparse matrix. First I create a class "Node" contains the index of entry, value of entry and two pointers to next row and next column. Second I find on Google that I need to create the class Matrix like this code below but I don't understand the meaning of Node **rowList and node** columnList. Why they use a pointer to a pointer there and how could I implement a matrix from that? Thank you so much.

class Node
{
public:
    int iValue, jValue;
    float value;
    Node *rowPtr;
    Node *colPtr;
};

class Matrix
{
    Node **rowList;     // rowList is the pointer to the array of rows
    Node **columnList;  // columnList is the pointer to the array of columns
    int rows, cols;     // number of rows and columns
}