What is meant by "alternation of generations"? Is it like the haploid phase is one generation (sperm, ova),

and the diploid phase is another (our bodies). I've often seen ferns and mosses quoted as having generation alternations,( there its clear that swimming sperm are clearly not like the organisms that they derived from).   So, something that has two different types of body(only as far as chromosome number goes), one with diploid cells and one with haploid cells has an alternation?  Therefore "ploidy" determines alternation and asexual reproduction would never alternate and multiploidal organisms would have multiple generations and they would be alternating (i.e. can't occur at the same time) ?  And/or does when meiosis occurs have everything to do with "alternation" (not alteration)? If so, then please describe how we may understand or actually see when this happens; is it quick or slow?
To CHVET: Please, How does this time in generation vary(i.e. making it "prevalent")?  Thanks