Home-on-jam is a form of passive radar homing used by missiles. It is usually added as an auxiliary guidance mode for missiles equipped with active or semi-active radar guidance.
Normally, when a missile is being jammed by a hostile electronic warfare system, it sees nothing but static and useless noise, since the returns that it should be homing in on are being drowned out by the jamming signal. However, the interference source itself can be detected quite clearly. The problem is that most radar guidance systems aren't designed to look for it. With just a minor software tweak, however, most radar seekers can be programmed to home in on the source of interference when they're being jammed. This effectively neutralizes jamming as an effective means of missile defense.
Many modern missile systems employ some sort of home-on-jam capability. It's most useful for anti-ship missiles, since warships frequently have effective jamming systems, but recently it's been used in anti-aircraft missiles, both SAMs and AAMs, too. Among the missiles known to include such features are Russia's SS-N-12 Sandbox, SS-N-19 Shipwreck and SS-N-22 Sunburn, France's Exocet MM40 and the United States' RGM-84 Harpoon. It's less common on anti-air missiles, but at least the AIM-120 AMRAAM and the MBDA Meteor are known to have it. Many other SAMs are rumored to possibly include home-on-jam capabilities, but this is difficult to confirm.
While home-on-jam largely defeats jamming as a missile defense measure, there are some weaknesses. In particular, some anti-ship missiles which use terminal evasive maneuvering abort such maneuvering and streak directly in when jammed. This renders them more vulnerable to active defenses and offboard decoy/jammer systems like Nulka. Also, it's possible to activate the jammers on a less valuable ship to lure an inbound missile away from a high-value unit. This might be used, for example, to pull a volley of missiles away from the carrier toward a much less valuable frigate while the group's escorting fighters, cruisers and destroyers continue to attempt to shoot down the inbound missiles.