I happens most easily with LME. Some kits/recipes have you putting all the lme in at the beginning of the boil. A 60 minute boil will darken,or caramelize it. Making it darker & can give that extract twang some talk about. Doing late extract additions with about 2/3 of the total amount lessen this effect & raise hop utilization in a partial boil.
Adding the remaining extract at flame out will help with this to keep color lighter,& flavors cleaner. And it's ok to do at flame out,since pasteurization happens at 162F,not boiling temp. So steeping for a few minutes will do the job with the lid on.