The is a demonstrative adjective and is similar to "that" The <noun> means "that <noun> which you spoke about earlier/which we both are aware of." or "the specific or particular <noun>"
Below is a link to the Skype call for Friday Fab 1st = Below is a link to the Skype call that you spoke about earlier/that we both are aware of for Friday Fab 1st
A is a quantifier and also means "an example of one"
Below is the link to a Skype call for Friday Fab 1st = Below is the link to an example of one Skype call for Friday Fab 1st - a/an always implies that there were other Skype calls on that day.