If I had one Rand for every time I have heard a coach ask a swimmer to streamline, I would be a very rich man. Streamlining is one of the first skills young swimmers are taught in learn-to-swim however it seems to be one of the most difficult skills for age group swimmers to do correctly.
Streamlining is about placing one hand on top of the other, extending the arms so the elbows are straight, squashing the head in between the arms to minimise resistance and keeping the body in a nice straight position as you kick hard through the water.
It is also about keeping the core body tight and straight, the legs straight and together, and the feet together with toes pointed. Make sure that you are streamlining correctly with your whole body every time you dive in or push off the wall.