Shield frequency

A shield frequency, shield harmonic, or shield modulation refers to the frequency settings of the shield emitters on a particular starship. If an enemy force were to obtain this frequency, they can modify their weapons to bypass the shields entirely. (Star Trek Generations) Shield functionality can be regained by remodulating the shields. (VOY: "Equinox, Part II")

In less than ideal sensor conditions, a ship can be identified by the pattern of its shield harmonics. (VOY: "Innocence")

In 2366, when the non-corporeal Koinonians drained antimatter from the antimatter pods to use it as energy to create their replica of Marla Aster, it was stopped by increasing the shield harmonics to match the antimatter containment effectively severing the Koinonian beam. (TNG: "The Bonding")

In 2371, Lursa and B'Etor were able to inflict severe damage on the USS Enterprise-D after learning its shield frequencies through modifications of Geordi La Forge's VISOR. They were then able to adjust their bird-of-prey's disruptors and torpedoes to pass through the Enterprise's shields. (Star Trek Generations)

The Vidiian starship that attacked the USS Voyager in 2371 was capable of continuously matching Voyager's shield frequencies, allowing them to clamp directly onto the hull. (VOY: "Fury")

In 2372, a shuttlecraft piloted by Tom Paris penetrated the shields of a Pralor starship by matching shield harmonics with those of the Pralor subspace defense field. (VOY: "Prototype")

In 2376, the USS Equinox successfully engaged the USS Voyager after learning its shield frequencies. Although Voyager remodulated its shields several times, each time the new frequencies were passed to the Equinox by their EMH, which had been switched with that of Voyager. (VOY: "Equinox, Part II")

This also explains how a ship can fire through its own shields. Their weapons would be set to their own shield frequency.