You will see or hear these terms when a company declares a dividend. 'Cum-dividend' means the shares carry the entitlement to the dividend and if purchased 'cum-dividend', the buyer will receive the dividend.
'Ex-dividend' means without the dividend. Therefore if you see the share price quoted 'ex-dividend', the dividend is earmarked for the seller, not the buyer. The market price will reflect whether the shares are 'cum' or 'ex' the dividend entitlement.