I read an English chapter in my school book and then as I was reading the stuff given after the chapter, I read this topic called 'Noticing discourse linkers and signallers' (I have attached the picture of the page I read).

I was able to understand what 'signallers' are but I was not able to even get a clue of what 'linkers' are. I did search it on the internet but I couldn't find a simple and understandable source. So, can anybody please explain this topic to me in as simple words and language as possible?

I will highly appreciate your efforts! :)