Complex sentence: The rotational periods of extrasolar planets are not known, but their proximity to their stars means that hot Jupiters are tidally locked (their orbits are in sync with their rotations).
Split into simpler sentences: The rotational periods of extrasolar planets are not known. However, for "hot" Jupiters, their proximity to their stars means that they are tidally locked (i.e., their orbits are in sync with their rotations).