'Get' is the equivalent of 'become' in sentences where this verb is followed by the past participle. For example, if you 'get dressed' or 'get washed', you put on your clothes or wash yourself, with the result that when you have done these things you are in the state of being 'dressed' or 'washed'. If you 'get drunk', you drink alcohol and end up in the state of being 'drunk'. You can also use this construction with inanimate objects, for example: 'My car got stolen', 'The window got smashed' or 'His leg got broken'. These mean that something happened which resulted in there being a stolen car, a smashed window or a broken leg. We use this construction when the person who did these actions is unknown or unimportant - the significant point is the result.

The form is really "get + adjective". The past participle of a verb can act as an adjective, as it does in this form.