Alan Jackson isn't the world's most original country singer, but he sure can communicate. His new record, What I Do, reaffirms his position as one of the genre's most believable balladeers (the heartbroken ''Rainy Day in June'') and, happily, provides him with a couple of chances to liven up. One number (''The Talkin' Song Repair Blues'') gives an auto mechanic a sly comeuppance. Another (''Burnin' the Honky Tonks Down'') is the barn burner its name suggests. Both are a hoot and a half.