stuff can refer to ANYTHING...
"clean the stuff off the table. i have stuff to do, i cant go out today. that store has cool stuff." here, stuff means [things] as activities, errands or objects, or ideas. "my nose is stuffy." (sniffly) as in swollen due to having the flu etc.
"the room was stuffy. the air is stuffy, open the window!" here, [stuffy] means the air is bad, musty, or stagnant. imagine if a house hasnt been lived in a while, or no one opens thier window.
"my head is stuffy. " here, stuffy means sick, not common.
"stuffing" can mean a type of bready-food often put in turkey and cooked, or the action of putting something somewhere without organization as in "she was stuffing the garbage until it overflowed." or without delicacy, (messy) "she stuffed her face with food like an animal"