This doesn't mean anything. It can be your internet connection. You don't use much resources when browsing pages and online games don't use browser resources. Still can be a connection problem. If the connection is not stable, as example, downloads fails.
I never used Free download manager but they have a page -
Free Download Manager - absolutely free download accelerator and manager - maybe you find better explanations there.
With the complements I use at firefox all I have to do is input the download address (http:// etc) and the program does the rest.