In simple terms when writing a lower case L the pen goes up and then comes back down and the strokes intersect at the bottom.
In what we're calling an open L there is no intersection of the pen strokes at the bottom...it's strictly goes up then pretty much straight back down.
If you're still unclear simply look at the example I posted and look at the bottom of the L in Presley...there is no intersection of the pen strokes...the loop is not closed.