What does 'kinetically accessible' mean in protein folding?
At some point during protein folding, there may exist lower energy states from a thermodynamic perspective (lower ΔG free enegy) which are actually unreachable in a given environment, because the required activation energy is too high. These are kinetically inaccessible states, conversely accessible state are those that can be reached.

For example, that could mean a protein could reach a lower energy state by unfolding a bit, but exposing hydrophobic amino acid even temporarily would require too much energy. So the lower energy state is not kinetically accessible.

This is somewhat similar to the concept of kinetic and thermodynamic reaction products in chemistry.