The Poke Radar was a feature in Pokemon Diamond, Pearl, and Platinum that helped you find Pokemon. Here's how it worked:
1. Get the Poke Radar: You receive it from a character in the game.
2. Find a shaking patch of grass: The Poke Radar's antenna would point towards areas where Pokemon were hiding.
3. Walk around the area: As you walk, the Poke Radar's sound would change, indicating if you were getting closer or further away from the Pokemon.
4. Find the Pokemon: If you kept walking, the sound would get louder and you would eventually find the Pokemon hiding in the grass.
The Poke Radar did NOT catch Pokemon directly. You still had to encounter them in the wild and battle them like normal.
Hopefully, this helps!