String concatenation means combining two String values into one. This is done with the plus symbol as in this example: 
    "first" + "second"
You can also concatenate other types of data to a String, such as:
    "two + two = " + 4
When you do this, the number 4 is first converted to a String, "4", then concatenated to the original String. You can put the thing to be concatenated either before or after the String. For instance,
    3.1415 + " is the value of pi"

String concatenation is one of the few operations which does not result in null if one of the input values is null. The only way to get a null result is if both inputs are null. If only one input is null, the result is the other, non-null input.