They just mean that a Pokémon is in the grass. There is a chance of it being a rare Pokémon, but it's not guaranteed. For example, look here. As you can see, using the PokéRadar is the only way to encounter Sentret on Route 202. However, Sentret only has a 12% chance of appearing when you touch the shining grass. Therefore, you are still able to encounter Pokémon normally encountered on the route, like Shinx and Bidoof, but you also have a small chance of seeing a Sentret. This is the reason you keep encountering common Pokémon: it's not a guarantee. I hope this helps!
EDIT: If you're looking for Kirlia, in Diamond/Pearl it can be found on Route 203 & 204 naturally. In Platinum it can be found naturally on Route 212 or with PokéRadar on Route 209. Alternatively you could catch a Ralts and evolve it.