The two frequencies are referred to as:
1. 200 Hz being the default primary
2. 50 Hz being the secondary
200 Hz is by far the best as it gives you a very high definition resolution and clarity of picture. The so called "cone" will vary by depth, similar to an inverted icecream cone. The shallower the water then the less area shown and vice versa for deeper water.
50 Hz will give noticeably less resolution and clarity though on the plus side, the icecream cone image will be over a greater area. You will not get the greatest returns on fish due this characteristic.
It's all a compromise. Also both these frequencies work much better in fresh water than salt water due salinity levels, sand and crap in the water.
Most decent sets allow you to "split" the screen so you can have half on 200 Hz and the other half on 50 Hz. If your set doesn't have this capability, then you will have to jump back and forth changing the frequenzies.
Not a Garmin/Lowrance/Hummingbird fan personally and prefer the more upmarket (read as more expensive) stuff like Furuno, Kodan, JVC etc. One of the few items about where $$$$$$$$ is worth it!
Having said that the cheaper brands are very popular and probably do their job pretty well for the price.
In the end, forget the AUTO function and learn to handle the MANUAL controls - you will get better results and understand the functions a lot more.