This means that the event or match has been postponed or cancelled.
If you have just place a single bet on that match then your stake will be refunded to you.
If you have placed a multi with a match or event that becomes VOID, your multi will keep going and that leg won't count. If all your other legs win, you will win the multi, but the odds from the VOID match won't be added to the overall multi odds. For example, if you had a 2 leg multi and both matches had odds of 2, meaning the overall odds of the multi would be 4 (2x2). If one of those legs become VOID, then your odds will go to 2 for the multi (based on the odds of the single matches) and you will still win the multi but at reduced odds.
If one of your other legs lose, then you will lose the multi as per usual and you will lose the stake.