When the 1,000 years are ended, Satan (the devil) will be made free from his prison \{in the bottomless pit\}.
The angel grabbed the giant snake (that old serpent). The giant snake is the devil (or Satan). The angel tied him \{with the chain\} for 1,000 years.
These four angels had been kept ready for this hour and day and month and year. The angels were freed to kill one third of all the people on the earth.