By good link, we mean that a profile that has lots of high authority links and does not contains any spammy links. To make it more clear I must say that there are two overarching qualities of a good link profile:
1. Lots of high value or high DA
2. High authority link backs.
Whereas, A bad backlink is a link that comes from an untrusted site that may contains lot of spammy links & does not have any DA or PA.
Since the inception of the Penguin algorithm in the year 2012, Google has been very strict on bad links. As after the introduction of this algorithm Instead of improving the rankings, Google actually penalized webpages that were associated with these kinds of bad links