[How to disable vector / pixels line]

Thank you ! 

[How to disable vector / pixels line]

Hi,

I'm new to Affinity designer and today, as I was working on it, it seems that I hit a wrong key on my keyboard. And so, I discovered a "vector / pixels line" on my screen. 

 

What could its use be (I don't understand in which context it could be used) and is there a way to disable it ? 

 

Thanks