DMS (Dimethyl Sulfide) is a cooked-corn or cooked-vegetable off-flavor caused by an insufficiently boiled-off precursor compound (SMM, S-methylmethionine) that's naturally present in malt, particularly pale and Pilsner malt, at higher concentrations than in darker, more heavily kilned malts.
Why the boil itself is the actual fix
SMM converts to DMS during the boil and needs to be driven off as a volatile gas through vigorous, open boiling — a boil that's too short, too gentle, or conducted with the kettle lid on (trapping the volatile compound and letting it condense back into the wort instead of escaping) all leave more SMM-derived DMS behind than a full, vigorous, uncovered 60-minute boil would.
Why pale lagers show this problem far more than dark ales do
Pale and Pilsner malt carries meaningfully more SMM than darker, more heavily kilned specialty malts, where the kilning process itself has already driven off much of the precursor before the malt ever reaches a mash tun — which is exactly why DMS shows up as a recurring troubleshooting topic specifically for pale lager styles like German Pils or Czech Premium Pale Lager, and rarely comes up as a concern brewing a dark porter or stout from a grain bill with far less pale base malt driving the issue.