This is dental caries, affecting both your wisdom tooth and the tooth before that, on viewing your image, it shows it can't be filled, it is better to get that tooth removed as it is damaging the before tooth so badly.

Yes it is a cavity which is spreading deeper. Your wisdom tooth is placed in too much of corner of your mouth. This area is not easily accessible to dentist's machine to do ideal work. Get a filling done after x ray if the dentist is able to reach your tooth well clinically. Last option is extraction whenever the tooth starts paining. You don't need to replace wisdom tooth after extraction.