Internal BGP (Border Gateway Protocol) explained

Hi rene,
Thanks for the lesson it was awesome, but I honestly don’t understand this image


If R1 advertise a prefix to R2, this will avoid this prefix because R2 see its own AS on the AS_PATH attribute.
Pls I need help, Its very difficult to me to understand if there is no a why?, so my next question is why do we need a Full-Mesh Topology?, I understand that point that SPLIT HORIZON is to avoid Loops within an AS, but this not imply to have a FULL MESH TOPOLOGY, or is just the rule of the BGP PROTOCOL.