A bastion in naval strategy is a heavily-defended area of water in which friendly naval forces can operate safely. Typically, that area will be partially enclosed by friendly shoreline, defended by naval mines, monitored by sensors, and heavily patrolled by surface, submarine, and air forces.
The Soviet Union had (and, of course, Russia now has) limited access to the world's oceans: her northern coast is ice-bound at least the majority of the year, and access to the Atlantic requires transiting the GIUK gap; much of her eastern coast is also ice-bound and requires moderately close approaches to either Alaska or Japan; travel from her southern ports involves transiting first the Bosphorus and Dardanelles, and then either the Strait of Gibraltar or the Suez Canal.
The Soviet Navy originally attempted to directly contest with the navies of NATO for control of the blue-water ocean. As the Cold War progressed, however, it became clear that the Soviets could not win a toe-to-toe fight in the deep water, and the information sold to the Soviets by the Walker spy ring in 1980s made it clear that the ballistic missile submarines, in particular, were very unlikely to be able to carry out their nuclear attack missions.
Realizing their vulnerability, the Soviets adopted a two-level approach. They armed their older, noisier, and less reliable "second generation" ballistic missile submarines with shorter-range nuclear weapons and deployed them as close as possible to the United States. Soviet submarine K-219, which suffered a catastrophic explosion and fire off Bermuda on 3 October 1986, was one such boat. Meanwhile, they used the information provided by Walker to build both dramatically improved attack boats such as the Akula-class, as well as more-survivable "boomers" such as the Typhoon-class armed with increasingly accurate and long-range missiles. Then they held those "third-generation" boats close to home, patrolling only near and under the Arctic ice cap. To secure the bastions, they also built large numbers of Sovremennyy- and Udaloy-class destroyers, whose primary mission was anti-submarine barrier and picket patrol.
The United States Navy practiced penetrating these bastions; one such attempt resulted in the 20 March 1993 collision between USS Grayling (SSN-646) and K-407 Novomoskovsk, a Delfin-class ballistic missile submarine. The collision was inadvertent and potentially catastrophic, but did demonstrate that US attack submarines were able to get rather near their intended prey.