Use of linear optocoupler to provide safety for analog input of BeagleBone Black
Most thermocouple signal conditioners (except the cheapest and nastiest) provide galvanic isolation both between the thermocouple and the output/power. 

If you have an grounded-junction thermocouple, then isolation is recommended. 

If you have an ungrounded junction sensor then you may be able to get away without isolation (but it's often bad practice, particularly in an electrically noisy environment). 

Isolation methods are optical, magnetic or capacitive. You can look for isolation amplifiers if you need one, and for many you'll need (an) isolated supply(s) as well. 

It's usually better to use a signal conditioner with built-in isolation because it will be made to the required accuracy and isolation standards (for example, capacitive coupling, which may be difficult for you to evaluate) and you'll get a guarantee of accuracy for the whole subsystem.