kovrilo – cover
tuko – cloth
Litotuko is a bedsheet, or a thin, easily washable piece of material that is used to separate the human from the less easy-to-clean parts of the bedding. It's almost always used below the human, and often a second one on top of the human (although a duvet cover can fulfill the same function here).
Litkovrilo is a blanket, or a thick piece of cloth that is hard to clean and/or wash, and is the main source of thermal isolation from the above (and sometimes bottom as well, it depends on how you make your bed). Kovrilo in general refers to all blankets, regardless of whether their intended use is bed, chair, or grass (piknikokovrilo).