it's going to be an app dependent measure of the buffer for the channel. just something to give you a visual clue about how the channel is streaming. When there are no bars lit, it's an indication that the stream isn't getting to you.
It's been shite for a week or so, before last week I'd never seen a low signal
Tuga? Posso te mandar mensagem?
On a phone that is the 4g signal strength. But in your case.. In this app if I would guess that would be the signal strength of the stream(channel). The issue is either related to your internet or to the stream provided by the app(maybe to many people using the he same channel/stream and not enough for everyone one). This would be my guess.