It's a common phrase used to ask for the location of something, but it's incomplete. You would need to add the object you're looking for after "taku".
Here are some examples of how you could use it:
* Kei hea taku kāri? (Where is my card?)
* Kei hea taku waea? (Where is my phone?)
* Kei hea taku kāinga? (Where is my home?)
Note: "Taku" translates to "my" in English.