Bluetooth Module

What module should i use? HC-05 or HC-06?
What does it mean when it can be set into a Master or Slave device?
I'm supposed to connect the Arduino to an Android Device.

In every Bluetooth connection there is a master device and a slave device. The master initiates the connection. The slave can only accept a connection.

When using an Android device, the Android device is almost always the master (to use as a slave normally requires additional software).

The HC-06 is a slave only device. The HC-05 is a master or slave device (user selectable).
This means you can use either a HC-06 or a HC-05 (in slave mode) to connect to an Android device.

Since the cost of the HC-06 is more-or-less the same as the HC-05 I would recommend going with the HC-05. It will give you more options later. I would also suggest buying more than 1.

There are many different HC-05s and HC-06s. Look for the original ones from Wavesen, these have the HC logo screen printed on the pcb. There tend to be slightly more expensive but they are well supported and the manufacturer makes full data sheets available.