‘Fully dilated’ means that your cervix is fully open so that your baby can begin to move down the vagina and be born.
When your labour begins, your cervix is either closed, or only one or two centimetres open. The contractions of the uterus open it further, little by little, until it is completely open. This is then termed the second stage of labour, and this lasts until your baby is actually born.
The point at which your cervix becomes fully dilated can occur quite quickly after the onset of strong, regular contractions, or can take many hours. 1
1 Mayes Midwifery