I think it means it is a right pain to get the tiles to look right and match heights with their neighbours if you have a wavy surface. It not flat, call in an expert.
You're going to put adhesive on top anyway, both to stick the tile to the wall and to raise it to the same level as those tiles next to it. If there is a lot of old adhesive on the wall you may use more new than is ideal to level everything, but I'd not worry too must about getting it 100% clear. That said I've only ever tiled over walls that didn't have anything more than wallpaper on it previously.