The button you click tells your computer how much memory IGB is allowed to use.
If you choose the low memory option and then later try to load more data than that amount of memory can handle, IGB may appear to stop working. To find out if this is happening, click the Help menu and choose the Show Console option. If IGB ran out of memory, you'll see a message that says java.lang.OutOfMemory.
If you choose the high memory option and your computer doesn't have that much memory, don't worry. Your computer won't let IGB take more memory than you computer really has. What happens then is that IGB will simply refuse to start running.