This is a great way to visualize this idea. The ocean surface can vary depending on the weather. Sometimes its smooth and glassy sometimes it rough and wavy.

Objects floating on the surface can vary in size and shape. Seeing those objects,the signal, depends their size, shape and color. A large red ocean liner can be seen from space and small floating grey soccer ball is almost impossible to see. A large ocean liner on a smooth glassy ocean has a high signal to noise ratio. A soccer ball floating on a turbulent wavy ocean has a low signal to noise ratio.