diff --git a/simplemind/chains/base.py b/simplemind/chains/base.py
index 279fdf5..41013c7 100644
--- a/simplemind/chains/base.py
+++ b/simplemind/chains/base.py
@@ -2,6 +2,23 @@ from abc import ABC, abstractmethod
 
 
 class BaseChain(ABC):
+    """Abstract base class for implementing chain operations.
+    
+    A Chain represents a processing step that can be executed on input data
+    and should be implemented by concrete classes to define specific behaviors.
+    """
+
     @abstractmethod
-    def run(self, input_data):
+    def run(self, input_data: str) -> str:
+        """Execute the chain's operation on the input data.
+        
+        Args:
+            input_data: The input string to be processed by the chain.
+            
+        Returns:
+            The processed output string.
+            
+        Raises:
+            ValueError: If the input data is invalid or cannot be processed.
+        """
         pass