Here's why:
* Lame: This refers to the fact that the Saint Sulpice church in Paris has a particular sundial with a gnomon (the part that casts the shadow) that is slightly off-center. This causes the shadow to fall "lamely" off the center line of the sundial.
* Saint: This is the obvious reference to the church itself.
The riddle is one of many that Sophie Neveu and Robert Langdon encounter in the book and movie, leading them on a chase to uncover the truth about the Holy Grail.