What IBGP Full Mesh really Means?

What underlies in the fact that BGP routers inside a singal AS must form IBGP full meshed topology is that BGP rules that BGP does NOT readvertise to internal BGP peers the routes learned from other IBGP neighbors.So if you don't full mesh it,some IBGP neighbor will never get the routes learned by other IBGP neighbors.
And what underlines in the fact that BGP does NOT readvertise to internal BGP peers the routes learned from other IBGP neighbors is to avoid routing information loops inside an AS.
This is the reason in essence.Don't confuse this with other related concepts such as Synchronization,Redistribution etc.