What does each function for vertex array buffer drawing in OpenGL do?

I have recently embarked on learning OpenGL for 3d graphics from this website. I understand that there are some very old tutorials out there for this library since it has been around for a long time. Most tutorials use the commands glBegin() and glEnd() to draw shapes. This is very simple and easy to follow in my opinion.

However, it has come to my understanding that this is supposedly outdated and I should be using glDrawArraysinstead. The problem is this, I just don't understand the code/concepts that are needed to draw using this method. The code is as follows:

void draw() {
    //i get this bit
    static const GLfloat g_vertex_buffer_data[] = {
        -1.0f, -1.0f, 0.0f,
        1.0f, -1.0f, 0.0f,
        0.0f,  1.0f, 0.0f,
    };

    //I stop getting it here
    GLuint vertexbuffer;
    glGenBuffers(1, &vertexbuffer);
    glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
    glBufferData(GL_ARRAY_BUFFER, sizeof(g_vertex_buffer_data), g_vertex_buffer_data, GL_STATIC_DRAW);
    glEnableVertexAttribArray(0);
    glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
    glVertexAttribPointer(0,3,GL_FLOAT,GL_FALSE,0,(void*)0);
    glDrawArrays(GL_TRIANGLES, 0, 3); 
    glDisableVertexAttribArray(0);
} 

Could somebody explain this code line by line and maybe explain the concept behind a buffer. Also, I'm curious, do the old draw commands still work in new versions and if so, is it bad practice to use them?

Please explain everything in as simple language as possible, I'm a complete beginner in this subject. TBH, I dont even know what a buffer is.