Strange is meant as "unusual" or "odd". These may not always be obvious at first. Like in dreams, where we accept the unusual without question.
But strange can be anything from seeing someone you haven't seen in a long time, finding a coin, going to a new store, forgetting where you put your keys, arriving late somewhere, etc... The trick is to recognise things that are different from the most normal situations. Then, eventually, to be able to recognise things that are so different from normal (i.e. "strange") that you must be dreaming, like when you find yourself dining with Cleopatra or that your bedroom has beaded curtains for walls and is a shack on top of a mountain in hell.
Because really, when is being sucked into the sky by a purple tornado filled with pillows and panda bears playing hopscotch and drinking scotch and liquid taffy normal? But the problem is that even the most absurd seems "normal" in a dream. So if you can learn to recognise small and large anomalies and inconsistencies in the real world then you are better prepared to recognise their counterparts in the dream world.