User Exit is a customize java code, that will help you to manipulate with the data before referring it as GOLDEN.
There are many class provided by Informatica for User Exit such as postLoadUserExit, postMatchUserExit etc.

Example: Suppose I have a business requirement to merge my record according to survivorship rules. Let say, I have a column name Amount, and I want to pick the highest value in the group of matched record for Amount. I can accomplished this task using post merge user exit.

How to add a UE in MDM Hub.

Step 1: Create your UE using Eclipse IDE.
Step 2: Export you java code into a jar file.
Step 3: Launch the MDM jnlp file.
Step 4: Enter you credential, select the ORS in which you want add UE.
Step 5: Under Workbench Utilities, select User Object Registry.
Step 6: Acquire Lock, select User Exits, click on Add User Exit.

Step 7: Provide the path of jar file and click ok.

MDM Hub will read all the UE classes is jar and display them under User Exit section.

P.S. you can only add one jar per ORS, but you can have multiple UE classes compiled in one jar file.