Plug-ins are used by web browsers (IE, Chrome, Firefox etc.) to allow them to deal with parts of some webpages and every now and then possible weaknesses that could result in security problems are identified in these plug-ins; at that time a plug-in needs to be updated to use its latest version that has the problem fixed.
If you are using Firefox then select Add-ons from the Tools menu, down the lhs of that you'll see Plugins so click on that, and at the top of the displayed list you will see Check to see if your plugins are up to date so click on that and follow the instructions.
If you are using another browser then you'd need to specify which it is.