The word "emerge" means to come out of, to be revealed, to come or appear into view. In this sense, it means to be revealed or to come into view, to be seen or known.
The sentence<em> "Periodical cicadas emerge in huge groups called broods" </em>uses the word "emerge" to talk of the cicadas that come out in huge groups, large numbers. Therefore, the word "emerge" is used here to mean that the cicadas come out to the surface.
