Baud rate is the speed of transfer of serial data, the higher the baud rate the faster the transfer of data but at the risk of the increased need of accurate timing between MCUs to cause data corruption.
You should be safe with something like 250000 baud but I have seen people get higher speeds.
Be aware if your using the Arduino UNO(or other 5v MCUs) connected to the RPi then you will need level shifting to convert to 3.3V of the RPi and this may introduce external limits to your maximum baud rate.
ADC conversion speed of UNO can vary at the possible loss of accuracy. See the thread here as it highlights speeds and accuracy values.