“Look!” The Greek word is idou (#2400 ἰδού), and it is used to get our attention (see commentary on Matthew 1:20). Here it is not spoken with great force, but to remind Jesus of the sacrifices the apostles had made. In this context, the meaning is close to “Look at what we have done. We have left everything and followed you.”
“…so what will we have?” Peter’s question is a good one, and one that all of us should be asking more. All of us are either like the rich man in Matt. 19:16-22 who hold on to worldly things and lose out on heavenly things, or we are like Peter who has “left everything” and will have great reward in the kingdom.