What is the relevance of "Low Signal" in battery usage stats?

Solution 1:

You’ll see “low signal” when you are using service in a poor reception area. When the phone needs to search for a signal all the time it uses more battery life. We normally see this on Home & Lock Screen and Personal Hotspot. The best thing to do to save battery in these situations is look for a way to get a better signal, or maybe even turn cellular off when you are in really bad reception areas. You can turn off cellular by going to Settings -> Cellular -> Off. With this off you can’t make calls, so you don’t want to leave this off all the time, but it can help in some situations.

"Better" is relative so the percentages would probably vary by situation.

Solution 2:

The percentage of battery used also factors in the amount of time you were using either the personal hotspot or the phone. Therefore, the percentage of battery use will also change, based on which service you use more frequently.

Popstar is exactly right: constantly searching for a signal requires more power than maintaining a good signal.

You could look at the difference by starting with a fully charged battery and performing tasks while connected to a poor signal and then repeating the same tasks (with a fully charged battery), for the same amount of time, while connected to a full signal.