This is called the Scotch - it's quite old but fell out of fashion in the 20th century. Kasparov famously 'revived' it in one of his matches against Karpov, and it's been seeing quite a lot of play ever since. Yeres is suggesting you play the Philidor defense - even older than the Scotch. I assume the point is to avoid a lot of theory (you dodge the Ruy Lopez, the Scotch, and the main line Italian game by playing ...d6 instead of ...Nc6). But since you asked about the Scotch, I'll line-dump for you and you can decide for yourself whether you want to mess with it.
the theory:
I feel like people play the Scotch because they want to make something messy, so, when I play it as black, I try to think my way to something cleaner.
Hi, I apologise in advance if this opening has been posted before (it probably has >.<") but how do I defend against it? I came across it in a game today as black, and lost spectacularly XD