Plants are like these things that grow out of the the land of the Earth, though come to think of it they grow in the seas and oceans too, but let's not get bogged down in technicalities.
Generally they have very restricted movement, due to being attached to the Earth by things called roots. Plants that are not rooted to the Earth are called Triffids, although they weren't really real and didn't exist. They were made up by this guy called John Wyndham.
Plants come in all shapes and sizes and colours and are classified in various ways. Some of them are called weeds, some are called flowers. There is speculation as to whether a weed can be a flower, and vice versa.
Very big plants are called trees.