can you help me to fix this Error

Error: Smarty error: unable to write to $compile_dir '/templates_c'. Be sure $compile_dir is writable by the web server user.

Backtrace:
/class/smarty/Smarty.class.php (1095)
/class/smarty/internals/core.write_compiled_resource.php (23)
/class/smarty/Smarty.class.php (1433)
/class/smarty/Smarty.class.php (1263)
/class/theme_blocks.php (132)
/class/theme_blocks.php (92)
/class/theme_blocks.php (36)
/class/theme.php (225)
/class/theme.php (62)
/header.php (72)
/index.php (43)

What is the actual error??? Link???