You pass it as a string, but then in your stored proc, you have to use "dynamic sql":
CREATE PROCEDURE [dbo].[spn_executeSQLExample]
(
@tableName VARCHAR(100)
)
AS
BEGIN
DECLARE @sqlStatement AS NVARCHAR(500)
SET @sqlStatement = 'SELECT COUNT(*) FROM ' + @tableName
exec sp_executesql @sqlStatement
END