Spam links refers, when you violates website terms and polices.
In simple words, when you go against website rules so this would be considered as spamming on site.
Majority of webmasters thinks, if you join any portal just for the sake of put your site links and you did it then that is spamming. Because you are not here for conversion with community, you are only here just to put your links whether it is good or bad site.
Hope this help you!