Hey
I can't tell you exactly what lecithin is, but i can tell you what i does to you in your edible.
It will make it stronger, and will make the effect appear faster, also remember to decarboxylate your good good before make the canna-butter.
You don't necessarily need to use soy lecithin, you can use like sunflower lecithin as well, i will definitely say it's worth using.
Hope this helps a little
Edit: I did not realize untill now, that Maitri already gave you like the perfect answer, by sending you that link.