How mesh geometry data (vertex coordinates) stored in glTF? I've read the reference guide about the file but I am really confused. Why the hell are there "min" and "max" values ?? A single vertex has only one coordinate array not min and max..

Also in the structure of the meshes why is there an "indices" reference to the "accessors" if we already put the indices to the "attributes" like "NORMAL: 2"?
The min and max values provide an outer bounding box for all POSITION data within a given accessor.  Accessors typically contain multiple vertices, for example all of the vertices of a particular primitive, and then min: [x, y, z] and max: [x, y, z] will offer the bounding box for that primitive.

For your second question, let's look at the mesh structure from BoxTextured.gltf:

"meshes": [
    {
        "primitives": [
            {
                "attributes": {
                    "NORMAL": 1,
                    "POSITION": 2,
                    "TEXCOORD_0": 3
                },
                "indices": 0,
                "mode": 4,
                "material": 0
            }
        ],
        "name": "Mesh"
    }
],


In the above example, mode: 4 is an enum, where 4 means TRIANGLES.  The indices are stored in accessor 0, and the POSITION data is stored in accessor 2.  So, accessor 2 will contain a list of vertex positions, but each vertex is allowed to be referenced by multiple triangles.  The indices in accessor 0 declare the triangles by indexing into the list of vertices.

Here's a sample of the start of accessor 0 from this model:

0 
1 
2 

3 
2 
1 

4 
5 
6 

7 
6 
5 
...


And this is a sample of the contents from the start of accessor 2 in this model:

-0.50000    -0.50000     0.50000 
 0.50000    -0.50000     0.50000 
-0.50000     0.50000     0.50000 
 0.50000     0.50000     0.50000 
 0.50000     0.50000     0.50000 
 0.50000    -0.50000     0.50000 
 0.50000     0.50000    -0.50000 
 0.50000    -0.50000    -0.50000 
 ...


In this manner, the sample model builds a cube by winding triangles around vertices at the corners.

For a more graphical explanation of this, check out the glTF Overview Card.