In 2-d imagine an upside down ice cream cone originating at the transducer and ending at the bottom of the lake. Depending on frequency, the width of the cone will vary. On 83 kHz, the rule of thumb is about 1 to 1 foot coverage at the bottom. On 200 kHz, your looking at about 1/3 the depth coverage on bottom. When a fish shows up, it is simply in the cone. The closer to the center of the cone the harder the returns will be. You can interpret all this and determine how far the fish is from the boat.
In DS mode, 455 kHz the sonar returns are wide side to side, but narrow front to back. Imagine a sheet of paper being dropped from the transducer into the lake on its edge.
There is much more to this, and it takes time on the water to really understand it.