I think you mean Test.IsRunningTest() method. From the docs available @ https://www.salesforce.com/us/developer/docs/apexcode/Content/apex_methods_system_test.htm#apex_System_Test_isRunningTest
"Returns true if the currently executing code was called by code contained in a test method, false otherwise. Use this method if you need to run different code depending on whether it was being called from a test."
This might be helpful sometimes to not execute certain blocks of you apex code. For ex: if you are calling a webservice in apex and you do not want it to the called when the testcase runs then you would simply wrap the callout with
// notice the NOT if(!Test.isRunningTest()) { //webservice callout }
This is used when you want to check if the code block is being called by a test class. For example you want to assign a specific value to a variable when test classes are running .
"Returns true if the currently executing code was called by code contained in a test method, false otherwise. Use this method if you need to run different code depending on whether it was being called from a test."
This might be helpful sometimes to not execute certain blocks of you apex code. For ex: if you are calling a webservice in apex and you do not want it to the called when the testcase runs then you would simply wrap the callout with
// notice the NOT
if(!Test.isRunningTest()) {
//webservice callout
}
If(Test.isRunningTest()){
variableOne = 2;
}