Cooking sugar is a delicate operation due to the high temperature, working with caramel at 180°C or 356°F can be very very dangerous. This why this is one of the only recipes on the site that you should make without any children around you.
In former times (not so long ago) patissiers had no thermometer, so to know the temperature of the sugar they dipped two fingers in water then in the cooking sugar, and then by touching their fingers together then releasing, they could tell by the behaviour of the sugar what stage it was at: thread, soft ball, hard ball, etc. That's the origin of the strange names of the stages.
Lemon juice drops are there to prevent sugar from brutal not wanted crystallization. I thougt it was a legend, so I don't put in, until it really happen to me when I was preparing a red sugar :
Pretty, but not usable...