If you are guileless, you are not a liar; you are innocent, and you might be a touch on the gullible side.
To be guileless is to be without guile. Guile is "deceit, duplicity and trickery." The young and uninitiated are the ones we call guileless, and they are the ones who often get stung by the more heartless among us. You might recall being a guileless freshman trying out for the school play, and being told by a veteran performer that it would be best to come to the audition for Our Town in a chicken costume, so you did.