What is the Expect100Continue property in System.Net.ServicePointManager?

I'm working on a system that manages the licenses of customers.

There is a feature on my system to return the customer's product key by querying Web Service .

And simple you inform the CNPJ it returns the a product key.

//Web Service
Gerencial.WebService.WSLicenca ws = new WebService.WSLicenca();

//Key
string ProductKey = ws.RetornaLicenca(CpfCnpj); 

But when performing this operation and the message of Error :

  

The request failed with HTTP status 417: Expectation failed

According to this article the solution to this problem would be to add the property Expect100Continue setting value as falso , which can be done by the configuration file or in the code snippet before querying through the Web Service.

Code

System.Net.ServicePointManager.Expect100Continue = false; 

Configuration

 <configuration>
      <system.net>
         <settings>
          <servicePointManager expect100Continue="false" />
         </settings>
        </system.net>
 </configuration>

Can anyone tell me what this is for?

Using this property the error has been fixed but I have no idea what this property does.