Ernst Haeckel once argued that "ontogeny recapitulates phylogeny." What he meant was that the individual development of a living organism sums up in itself the development or evolution of a species as a whole. He would have claimed, for instance, that by studying how a human embryo grows and develops, biologists can get an idea of how human beings as a whole evolved. The individual reveals the process in miniature and over a much shorter period of time. This theory has been discredited in the field of biology.
The idea of "ontogeny recapitulates phylogeny" does, however, still carry some weight in theories of language development. In this area, the phrase refers to the language development in a child mirroring or summing up the language development of early peoples. We can see how a child learns language, starting with a few simple words and eventually adding more and more and learning how to put those words together to create phrases and sentences.
We also see how the child learns grammatical and syntactical rules through imitation and applies them (with mistakes at first, but gradually more and more effectively and accurately). Some language theorists argue that this process of language learning in a child tells us something about how human beings first learned language, starting gradually with some expressions and then building into sentences, phrases, and the rules of grammar and syntax.