# Let us rejoice

Here "us" refers to all of God's servants.

# give him the glory

"give God the glory" or "honor God"

# wedding celebration of the Lamb ... his bride has made herself ready

Here John speaks of the joining of Jesus and his people together forever as if it were a wedding celebration.

# Lamb

This is a young sheep. Here is it used symbolically to refer to Christ. 

# has come

Existing in the present is spoken of as having come.