I have had a glance at the link posted by the voluntary worker above - I think you might find it a little dense to assimilate, so I though I would help you with a simplified explanation.
All your phrase means is that the stages that a foetus goes through during its existence in the uterus of the mother [i.e. the stages in embryological development] are almost like an extremely shortened repetition of the processes that occurred over millions of years during evolution, which led to the highly complex and intricate anatomy and physiology [structure and function] of the human adult.
In other words, each stage of embryology resembles simpler organisms on earth, starting from the simplest immediately after conception through more and more advanced stages leading eventually to the full-term baby.
Hope this makes it a bit easier to understand; hopefully you can manage with a few "grandmum's seven portions" RATHER THAN having to consume a full roast beef, Yorkshire pudding, roast potatoes, Brusell sprounts, peas and carrots [the brain, of course needs energy, but almost exclusively in the form of glucose!]
"My Q was not on nutrition!" he/she replies, particularly if OP is a "she", cos we guys love our tuck-box.
Have a great day & be safe!
Ontogeny is the process of development that an individual organism goes through. In humans, - as macpatgh-Sheldon says - that includes the growth of the foetus from a tiny ball of cells, through a stage with gill slits that looks a bit like a fish, then one with tiny limbs that looks like a lizardy-type thing, until it takes on the clear appearence of a primate. What the theory of Haeckel says - linked by SlaveofAll - when we look at this process, what we are seeing is a repeat (or "recapitulation") of the whole evolutionary history of that species (which is what "phylogeny" means).
It's a very attractive theory, but it is not exactly true. For example, there are many examples where these apparent parallels turn out to be independently evolved solutions to similar problems, rather than derived from the same ancestor. However, any comparison of the anatomy of different species can give us clues about their relatedness, so it can sometimes be helpful .