What's the meaning of 777? I heard it means the completion.
From the standpoint of the Bible, the number 777 has no specific meaning. The number seven often seems to stand for completion. (See What Does the Bible Say About..The Number Seven? for my previous answer about the number seven.) But there is no reason why multiplying it by 111 would retain that meaning, or any meaning. Just as the number 666 only had a specific meaning to Christians in the first century (as representing a living man, probably Nero Caesar), so any other number has no meaning unless assigned a specific one. The Bible never assigns any meaning to seven hundred seventy seven, although it did assign a meaning to six hundred sixty six.
The only time 777 appears in the Bible is the age of Lamech when he died (Genesis 5:31)