Laurence Templeton is the pen name Scott uses in his "Dedicatory Epistle" to Ivanhoe. We get into this more in "In a Nutshell," but Scott didn't come out as the writer of his enormously successful and influential novels until thirteen years after publishing Ivanhoe. He signs the fake letter at the front of the book with this name instead of his real one.
Laurence Templeton writes to another fictional guy, Dr. Jonas Dryasdust, to explain why he dares write a novel about such a distant historical period as the 1190s. Templeton acknowledges that he has been successful with his recent histories and that the 1190s is a very long time ago. Clearly a lot of the stuff in Ivanhoe will be outright made up. But Templeton justifies his decision: even if the content of Ivanhoe is not 100% (or even 50%) accurate, it will still capture something about the spirit of the times. After all, Ivanhoe is not supposed to be a dry history book. It's a romance, with all of the unlikely plotlines that go along with that description. For more on Scott's justification of Ivanhoe's weirdness, check out our "Detailed Summary" of the "Dedicatory Epistle."