Railflex is Tyrolia's Rail mounting system. Takes the drilling out of binding mounting (on factory skis). It also helps maintain a constant flex on the ski by eliminating the "dead" spot under foot. The DIN settings are also made more consistant by allowing your heel and toe pieces on a rail to "float". Whenever you flex your skis on a more traditional drill-in binding setup, the forward pressure increases and decreases, which can result in higher or lower effective DIN settings.
Basically, you adjust your binding's length according to your boot sole length on the rail, then you slide the whole thing onto the rail track, tighten one big screw, adjust DIN, check forward pressure, and off you go.
They discountinued the original railflex system and made a new Railflex 2 recently. If you do happen to get a ski with railflex mounts, make sure you have the right generation of binding as they are not compatible with each other (gen 1 and gen 2 railflex).