For unicast routing, LDP advertises labels for each prefix in the IP routing table. LSRs use LDP to send messages to neighbors advertising the prefix and label assigned to it.
Advertisements are triggered when new routes are injected into the IPv4 RIB. The local label applied to the prefix by the router. Basically the router is saying to reach this prefix across the mpls, label your traffic with the corresponding label thats being advertised with that network.
A Label Switched Path is the complete set of labels at each LSR used to get to an MPLS destination.
LSPs are unidirectional.
LSRs in the MPLS cloud need to use the same IP routing protocol to exchange routes and ultimately apply labels to routes that are learned from each other.