Hello eatinpie and welcome.
The picture is poor quality, but the knot looks like a Turks head knot that is tied around one of the strands with the end of the other. Not the best choice for a necklace that needs to be adjustable, if that is your purpose for it. Might be a three part x 5 bight(?). It will be bulky and hard to get it to have good friction on the line.
A suggestion: You might consider using 2 double fisherman's knots.
BTW, please post in the most appropriate board. This is more decorative in nature. Thanks.
SS