Only a handful of these so-called x-holes exist in the known universe. An x-hole is leads to other x-holes simultaneously - an immeasurably important object for strategical purposes.
To jump through an x-hole, simply select your destination via dropdown menu and click Jump in the Commands window.
At the beginning, an x-hole jump will cost you over 2000 APs. This value will decrease if you gain maneuver skill.
Similar to the x-hole is the y-hole. Search Y-Hole for more info.