A boat tows the camera behind it. The camera is usually called a fish . It sends out the sound and measures how long it takes for the sound to come back to it, and how loud it is. A computer keeps track of the information and can makes pictures of the bottom of an ocean, lake, or river.
Some satellites have special cameras that can take pictures of the Earth.
The brighter the point, the higher the ground is at that point. The brighter the point, the deeper the water. Those are canyons. Terrestrial refers to the Earth, so Terrestrial Remote Sensing means gathering information about the Earth from a distance. Remote sensing is also used to gather information about planets and the stars.