"Then another portent appeared in heaven: a great red dragon, with seven heads and ten horns, and seven diadems on his heads."
The Wikipedia entry for chapter 12 of Revelation says that the Greek word for dragon appears nine times in the Bible, only in Revelation. Over and over again throughout history when Christianity has conquered paganism the gods of the prior culture are portrayed in artwork as, or referred to as, dragons.
In Ellicott's commentary on Bible Hub.com, the numbers of heads and horns on this figure are explained this way: "There the seven heads are explained as seven kings, and the heads here are crowned; the ten horns are also explained as ten kings. The sevenfold kingship and the tenfold power of the world are thus described as belonging to the dragon. The picture here, as the picture of the wild beast in Revelation 17, represents, as concentrated into a single hostile form, all the varying forces and successive empires which have opposed or oppressed the people of God, and sought to destroy their efforts for good..."
Mathew Henry's commentary at the same site says, of the dragon, "A dragon is a known emblem of Satan, and his chief agents, or those who govern for him on earth, at that time the pagan empire of Rome, the city built upon seven hills.As having ten horns, divided into ten kingdoms. Having seven crowns, representing seven forms of government."
Meyer's commentary at this same site goes into far more detail about the numbers of heads, horns, etc in this verse and how they have been interpreted over time by different authors. I think it's important to read a lot of different input when we are dealing with puzzling imagery like this that is not really explained in detail.
Both bibleref.com and gotquestions.org are also great resources for help with Bible verses whose meaning is not immediately apparent or those which use specific imagery that is unfamiliar, as in much of Revelation.
A Prayer for Salvation from Christ:
Lord
Jesus, I confess my sins and ask for your forgiveness. Please come into
my heart as my Lord and Savior. Take complete control of my life and
help me to walk in Your footsteps daily by the power of the Holy Spirit.
Thank you Lord for saving me and for answering my prayer.
A more detailed step by step process:
https://www.wikihow.com/Accept-Jesus-Into-Your-Life
