This means that they do not completely solidify. Essential oils don't freeze per se. If your essential oils are pure, they won't freeze. When your essential oils freeze, it means they contain a lot of water.
They will then solidify due to water. Allow them to thaw slowly before using them. Putting essential oils in the freezer is perfectly safe. Constant temperature and lack of light help preserve oils.
There are some oils such as rose, thyme and others that may have natural components that solidify or crystallize in the freezer. All liquids can freeze at a certain temperature, even essential oils. If your essential oils freeze, allow them to return to room temperature naturally before attempting to use them. Do not attempt to heat them to return them to a liquid state.