Question

What is a pokeradar how do you get it?

Answer 1

The Poke Radar is something that was only available in Gen IV (DPPt), and it's used to seek out Pokemon hiding in tall grass. It's also used to "chain" Pokemon in order to increase the chances of finding a shiny Pokemon.

You get it after you've been upgraded to the national dex (for seeing every Pokemon in the Sinnoh Dex) from Oak, and then Rowan will give you the Poke Radar.

Poke Radar - Bulbapedia will explain more.

Answer 2

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 tall grass.

That's what it is :P

After the player has seen every Pokémon in the Sinnoh Pokédex, they will be able to meet Professor Oak in Professor Rowan's lab in Sandgem Town. After Oak upgrades the Pokédex to the National model, Rowan will give the player the Poké Radar but will not explain it very thoroughly. In Pokémon Platinum the opposite-gender character, Dawn or Lucas, do however - when spoken to they give a practical demonstration on Route 202 if the player has received an Eevee from Bebe in Hearthome City.

That's how to obtain it.

There is also a version of this in HG/SS called the Pokewalker

Source: Bulbapedia