It’s a speculation, a theologoumenon, and it isn’t dogma. I bet that more than half of the Orthodox don’t believe it.
It is that, after death, your soul goes past toll-houses, each about a special sin, and demons there will start to accuse you of (unconfessed) sins in that area, while your guardian angel acts as your advocate. If they find fault and sin in you, they will drag you to hell. If not, you will pass to the next until you made it through all 20 toll houses. During these days there is much prayer from your relatives on earth, which somehow help.
Again, this is not an obligatory belief in Orthodoxy.