The syntax used for the parameter is invalid for the parameter type. Every tool parameter has an associated data type, which describes the contents of the variable. Some simple data types are string (any set of alphanumeric characters), Boolean (a true/false value), and long (an integer value between -2,147,483,648 and 2,147,483,647). In addition to these simple data types, there are dozens of other data types built specifically for data found in ArcGIS, such as coordinate system and extent.
Open the tool's help page, and look at the last column in the Syntax table to determine the datatype of the parameter. For information on how this datatype can be specified correctly, see data types for geoprocessing tool parameters.