After analysis of your library, if Serato DJ Pro detects a corrupt file - it will display the corrupt status icon, the broken disc (or lightning bolt).
There are several different types of corruption, so to investigate the reason further, hover your mouse over the status icon in the leftmost column and at the bottom of the screen there will be a longer description.
It's important to periodically analyse your library as you add new files, and remove any corrupt files, and if necessary, try and 'fix them':
Please read this article about analysing your library
Please read this article about re-encoding corrupt files