To strum is to play a guitar (or ukulele, banjo, mandolin) by dragging your fingers or a pick across several strings at once, so they all sound together. That is the key idea: strumming is many strings in one movement, while 'pluck' or 'pick' is one string at a time. It is the way most beginners play — you hold a chord with your left hand and strum with your right, up and down, keeping the beat. The word also carries a feeling of easiness and no great effort. 'He sat on the step strumming his guitar' does not mean he was performing; it means he was passing the time. Because of that, people sometimes say it a little dismissively — 'he just strums a few chords' means he is not really a skilled player. It works as a noun too: 'give it a strum', 'a slow strum', and 'strumming pattern' is the ordinary term for the rhythm of your up-and-down hand. Note that it is only for stringed instruments you hold; you do not strum a piano or a drum.