This means that the event or match related to a leg in your bet has been cancelled, abandoned or postponed.
If the leg of your single bet is resulted as VOID, then your stake will be refunded.
If you have placed a multi bet with a leg is resulted VOID, your multi will remain LIVE and that odds contribution of the voided leg will revert to 1.00, when calculating the combined odds of the multi.
For example, if you had a 2 leg multi and both legs had odds of 2.00. The combined odds of the multi would be 4.00 (2.00 x 2.00). If one legs becomes VOID, then the combined odds will revert to 2.00 (2.00 x 1.00). You win the multi bet but at reduced odds.
If one of the legs of your multi bet loses, then you will lose the multi bet and related stake.