Strumming is a way of playing a stringed instrument like a guitar, banjo or ukulele. You use your fingernail or a plectrum to sweep past several strings in order to play a chord. This is different from plucking, which only involves one string at a time.
Strumming is an important part of rock, pop, jazz, funk and country music! On the guitar, an upstroke is when you strum upwards towards the ceiling. A downstroke is when you strum down towards the floor.