From Bulbapedia, the community-driven Pokémon encyclopedia.
The Poké Radar (Japanese: ポケモントレーサー Pokémon Tracer, ポケトレ Poké Trace for short) is a key item in Generation IV that is used to seek out wild Pokémon hiding in long grass. Like the Vs. Seeker, its battery must be charged after use by walking around. It takes fifty steps to fully charge the Poké Radar.
After the player has seen every Pokémon in the Sinnoh Dex, they will be able to meet Professor Oak in Professor Rowan's lab in Sandgem Town. After upgrading the Pokédex to the National model, he will also give away the Poké Radar, but will not explain it very thoroughly.
It can only be used in the standard tall grass, while on foot, at which point a tune will begin to play, and one or more patches of the long grass will begin to move. Patches that shake more violently than others are often rarer Pokémon, including Pokémon exclusive to the use of the Poké Radar. When a player walks into one of these moving patches, a battle with a wild Pokémon will instantly begin.
If the Pokémon that is found is knocked out or captured in a Poké Ball, a chain will begin. These chains consist of multiple members of the same Pokémon species encountered one after another. The only catch is this: a player must not encounter any Pokémon just by walking through non-wiggling grass, only by walking into the grass that shakes. Therefore, it is recommended that Repel is used in order to ensure this and achieve a higher chain. Entering the same type of grassy patch the chain was started in that is the farthest away increases the chances of meeting the same kind of Pokémon consecutively. Long chains increase the chance of finding a Template:Shiny2 Pokémon, which is indicated by the patch of grass glowing white twice rather than shaking.
One of the many functions of the Pokétch is to display the current chain and the three best chains made so far.
Note that the statement "The grassy patch remained silent..." may occur. This is due to no chains found by the Poké Radar. Upon walking another 50 steps, the item may be used again and may work.
List of Radar-exclusive Pokémon
On Bulbagarden forums