i seem to find this alot in samples

Code:
    D3DXMATRIX matProj;
    D3DXMatrixPerspectiveFovLH( &matProj, D3DXToRadian( 45.0f ), SCREEN_WIDTH / SCREEN_HEIGHT, 0.1f, 100.0f );
    g_pd3dDevice->SetTransform( D3DTS_PROJECTION, &matProj );
but i dont understand what this is for, is there any straight forward explanations for this and what its use is for, from what i see, it is some help for moving the scene? seems most calls to move change maxtrix perspectives, but still dont get it tbh,

thanks,