Can someone give me the meaning of this biblical quote in layman's terms please?

I think that in very basic terms it means that there is room in heaven for anyone who wants to go there. A normal house is made up of many rooms but God's house is made up of many mansions, i.e. so many many times bigger. If it was already full or it was too late, Jesus would have said so. And he ascended to heaven to go and get 'rooms' ready for anyone who wants to live there in eternity.