Voronoi textures work by scattering points and connecting their edges to form a kind of mesh, then generating patterns based on distances within that mesh. The crackle pattern is created by measuring how close a shading point is from one of the edges. This results is a pattern of cracks following the random layout of the "edges" of the point mesh.
So to get the crackle pattern, you should first set the node to "distance to edge"
If you need an actual hard-edged crack pattern rather than just the distance to the edge, you can use the math node set to "greater than" to find areas that are more than a specified distance from an edge:
Finally, since this example uses a plane with UVs, you can change the texture from 3D to 2D. This will prevent the pattern from getting thicker where it intersects corners of the pattern in 3D space.