Iso-stars were small inter-dimensional points of energy that emanated isolinium radiation and allowed a starship to transport between the primary universe and the mirror universe.
The method in which a ship is brought into the opposite universe is explained as simply scanning the phenomenon at close range. A bolt of energy is then thrown out and attached to the ship, transporting it to the other universe.
In 2375, the ISS Phoenix-X was transported to the primary universe, where they were confronted with the crew of the USS Phoenix-X. ("Pure Evil")
Two years later, when Lex and James stole the Gotens symbiont from Commander Night Gotens, and replaced Night's symbiont with the Seifer symbiont, both Lex and James escaped through an iso-star and into the mirror universe. Since then, the now Commander Seifer had been driven into finding these stars in order to get to the mirror universe and find Lex and James. ("Secret Shuttles, Part II", "Avalon Battlefield, Part I")
The Nega'Jem and the Sleri were from the mirror universe, and had crossed over when the Vector 3 of the Phoenix-X had returned from a trip from the mirror universe, using an iso-star. Later, the crew of the Phoenix-X would attempt to force the Nega'Jem back into the mirror universe with the use of an iso-star. But when the Nega'Jem starship Zidane opened fire and destroyed the iso-star, a large shockwave was sent out and ruined space for a whole sector. ("The Nega'Jem, Part II")