An LDP implicit null label causes penultimate hop popping by a downstream router. LDP is a protocol that distributes labels for MPLS forwarding along the shortest path calculated by an IGP. An implicit null label is a special label value of 3 that indicates that the downstream router does not need a label to forward the packet to its destination. When an upstream router receives an implicit null label from a downstream router, it removes (or pops) the label from the packet before sending it to the downstream router. This process is called penultimate hop popping (PHP) and it reduces the load on the downstream router, which can forward the packet based on its IP header or another label in the stack59.

References: MPLS Label Distribution Protocol Commands - Cisco, Solved: Implicit null and Explicit null - Cisco Community