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! :)

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  [1]: https://i.sstatic.net/98JTz.png
  [2]: https://i.sstatic.net/E8OkF.png