It means that the son you will have, if not exposed to any horrors of life, will become a great king. If he is exposed to the horrors of life, he will become a great spiritual leader.
This is my last life on Earth
Queen Maya has a dream of a white elephant and became pregnant by the dream. She wakes up feeling happy.
The four horrors of life
The King and Queen Maya ask the astrologer what the dream means.
On her way to her mother and fathers house to give birth to the child(which is custom there) she orders her slaves to stop so she can rest. Then, as she feels the pains of birth, she gives birth to Siddhart Gautama while standing, holding onto a branch. The newborn walks seven steps and declares"This is my last life on earth!".
Siddhart sees old age, sickness,death, and a emasciated ascetic , something that he never saw when he was locked up in the palace.
While Siddhart is meditating to find the way to end suffering, he is tested by Mara in many different ways that all prove ineffective in disturbing Siddhart from his meditation.
You will never reach spiritual enlightemnment.
After his trials, Siddhart becomes the Buddha which means "the enlightened one".