Type 'TabletsDAL.tblUser' in Assembly 'TabletsDAL, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' is not marked as serializable.

I am new here plz help me figuring out this error. thanks in advance

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.  Exception Details: System.Runtime.Serialization.SerializationException: Type 'TabletsDAL.tblUser' in Assembly 'TabletsDAL, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' is not marked as serializable. Source Error: 
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
Stack Trace: ...