It means while you control an Ultimate Tyranno, all other monster you control cannot attack until your tyranno can't attack anymore. Let's say your opponent has 3 monster. Your ultimate tyranno can attack all monster your opponent control, after that it cant attack anymore. then, your other monster can attack
Is the attack of this card optional or not? Although it says "it must attack first", that I guess doesn't mean that it is forced to attack. In other words, it says you can pass if you don't want to attack but if you want, you must attack with this card first.