Block is a new mechanic that acsociated with Metallic type. They often stated like: 'When this card is in face-up DEF position, It can target to 1 face-up monster opponent's control with ATK is lower than DEF of your monster, the targeted monster cannot declare an attack or change its position.'
You can UNBLOCK 1 targeted monster by switch your monster into face-up ATK position, thus you can switch it into DEF position again to BLOCK another monsters.