What is meant by "alternation of generation"?

Answer 1

In zoology it refers to organisms which have two distinct morphologic forms, where the offspring of each form is the opposite form. Typically one of the forms reproduces sexually, the other asexually. For example, some coelenterates have a free-swimming medusa (jellyfish-like) stage, and a sessile polyp (anemone-like) stage. The mature polyps reproduce asexually by budding off medusae. The medusae once mature reproduce sexually, shedding gametes into the water, and the resulting embryos settle to the bottom, attach to the substrate, and grow into polyps, which once mature produce more medusae ...

Answer 2

In certain species of organisms, like fungi

They produce a diploid generation called sporophytes, which produce haploid gametes by meoisis, which are spores, the spores can then fuse to make a diploid organism which is a sporophyte or develop in multicellular haploid organisms called gametophytes.