A thneed is a fine something-that-all-people-need, knitted from the tufts of a truffula tree. It can be assumed that thneeds were created with Seussian physics in mind, and as such can be used for almost anything! As you can probably tell from the image, they don't seem to make a lot of sense by most other universes' standards...
At first hand-knitted by The Once-ler, then mass-produced by his factory to meet public demand, you could say thneeds (and their creator) are the reason truffula trees went completely and tragically extinct - as well as ruining the truffula valley and making it virtually uninhabitable due to pollution.
They also come in a cheerful shade of pink.
☞list of uses
For your convenience, here is a comprehensive list of all (known!) uses of a thneed. By all means, feel free to experiment - it's said that thneeds have a thousand uses...
shirt
sock
glove (mitten, or boxing glove!)
hat (three styles - one looks like a weird floppy top hat, one looks like a... um... inverted mushroom... and the other... looks like a... pile of... thneed. SUPER TRENDY HATS DEFY EXPLANATION OK)
carpet
pillow
sheets
curtains
covers for bicycle seats
vest
hammock
toothbrush holder
canary nest
thneedle soup (it's a cure-all!)
duster
carburetor rust remover
windshield wiper
viper trap
can be smoked in a pipe
reusable baby's diaper/nappy
suit
parachute
butterfly net
exercise mat (and... other exercise-related uses too!)