A more thorough explanation:

Definition: A slot charter is a type of charter for one or more slots on a container vessel. Each slot can accommodate a 20-foot container. It is a form of vessel-sharing agreement.

Example: If three carriers all serve the New York to Rotterdam route and each devotes one vessel to the route every three weeks, they can implicitly (with slot charters) join forces and each offer weekly service. This means that two or more carriers can combine their capacities and offer more frequent service on their routes.

Explanation: A slot charter is a way for carriers to share the capacity of a container vessel. It allows them to combine their resources and offer more frequent service on their routes. In the example given, three carriers can offer weekly service by each devoting one vessel to the route every three weeks and implicitly joining forces with slot charters.