Suds are the soft white bubbles that sit on top of water when you put soap or washing powder in it — in a sink of dishes, in a bucket, on your hands, in a washing machine window. The word is always plural: we say "the suds are", never "a sud". You cannot count them, but the word still takes a plural verb, which surprises learners. It sounds a little homey and old-fashioned; a shop label or a science book would more likely write "foam" or "lather", while "suds" is what someone says while standing at the sink. In American English there is an extra, joking sense: suds can mean beer, because of the foam on top — "a cold suds after work". That one is slang, mostly older and mostly male, and your students should recognise it rather than use it. There is also a verb, to suds something, meaning to wash it with soapy water, but it is rare and you can safely ignore it.