Ok, so all metals are made up with crystals. Thousands of them! If metal parts are made up of lots of crystals, there are boundaries between each crystal, which means the metal will break at these boundaries first.
If we manufacture a part really carefully using something called investment casting, then we can grow only one crystal, so the part wont have all the boundaries that normal metals do.
With no boundaries, the metal is really strong – it can even operate past the melting temperature of the non-single crystal material! And that’s really important for jet engines because the hotter the core, the more efficient the engine can be (so you can burn less fuel).