table = AgentReadTable($node, "System.Processes");

will read agent table (System.Processes in this example) into variable. It will be object variable of class Table. Then you can walk through table using it's methods. For example, to read value of column "NAME" in first row you can use the following code:

colIndex = table->getColumnIndex("NAME");
value = table->get(0, colIndex);

You can iterate through table using rowCount attribute as stop indicator:

for(row = 0; row < table->rowCount; row++)
{
   value = table->get(row, colIndex);
}

Best regards,
Victor

P.S. If you meant transformation script of a table DCI, then you don't need to read table - variable $1 will hold table object.