what exactly is google penguin?

Penguin was Google's update aimed at manipulative links, bought links, spammy directories, exact match anchor text everywhere. It first rolled out in 2012 and these days it's just part of the core algorithm rather than a separate thing. The practical bit: it mostly ignores bad links now instead of punishing the whole site. So unless you've been buying links, it's not something to spend time on. Build a few real links and stop worrying about it.