Fractals of the Mists is a high-end dungeon. It’s built of an array of mini-dungeons named fractals. Each fractal is built as a distinct, separate experience with themes, mechanics, and art specific to that fractal. Fractals of the Mists is a five men dungeon accessible to all players (your party can be any level), but it will be very challenging to low levels players.
You access the Fractals of the Mists in Lion’s Arch by entering a new asura gate (you can find location on the screenshots below).
Once you enter the portal you’ll be taken to the Mistlock Observatory, and faced with a set of three randomly chosen destinations. Every other round before you return to the Observatory you’ll be faced with a bonus “boss fractal”. To make Fractals of the Mist even more challenging, every time you re-visit the hub, the dungeon scales up in difficulty and becomes more rewarding.
With the Fractals of the Mists, ArenaNet will introduce the first Ascended items and Infusions.
Source: GW2 Official page