A synecdoche is a literary device in which the part of something represents the whole or vice versa. For example, people often refer to a car as "a set of wheels". Here, the wheels—part of a car—are being used to represent the whole vehicle.

In Beowulf there's a very good example of synecdoche in lines 32-33:

A ring-whorled prow rode in the harbor, ice-clad, outbound, a craft for a prince.

This is an example of synecdoche because the "ring-whorled prow", which is only part of a ship, is being used to represent the whole vessel. As a prow cannot, by itself, ride in a harbor—or anywhere else, for that matter—the reader is able to infer that the author is referring to a ship. Once we know what a prow is (that it's part of a ship), we're able to understand this particular excerpt's meaning.