Meaning, they should form a 3-bundle of non-intersecting geodesics orthogonal to a series of spacelike hypersurfaces (hyperslices).
[1] Sometimes, the additional hypothesis is added that the world lines form timelike geodesics.
This is an attractive viewpoint, but in general no such family of hyperslices will be physically preferred.
The Weyl hypothesis can be understood as the assumption that we should consider only cosmological models in which there is such a preferred slicing, namely the one given by taking the unique hyperslices orthogonal to the world lines of the fluid particles.
The condition that the congruence corresponding to the fluid particles should be hypersurface orthogonal is by no means assured.