The kanji 訳 means “reason,” “circumstances,” or “meaning.” As a grammar point, it is used in a number of different ways to express or disaffirm a logical conclusion based on context or reason. Used in this way, the word is very often written in hiragana.
週に３回以上ジムに通っていたら会員になったほうがお得なわけです。
If you’re going to the gym more than three times per week then it would make sense (it is the logical conclusion) to become a member.
嬉しそうな顔をしているけど、入試に合格したわけなんだね？
You look happy—that means you’ve passed the entrance exam, right?
そういうわけで、すぐに来たんだよ。
That’s the reason why I came immediately.
Partial Negation (わけではない)
When used affirmatively, わけ expresses that something makes total sense (“No wonder!”). Couple it with the negative auxiliary verb, however, and we have a very common and useful phrase that can be used to stop the listener from inferring the natural conclusion from the context. Its closest English translation might be, “that doesn’t mean…” or “it is not the case that…”
大学でロシア語を勉強したけど、ペラペラと話せるわけではない。
I studied Russian in university, but that doesn’t mean that I can speak it fluently.
会計士資格を持っているんだけど、数学が得意なわけではありません。
I hold an accountancy qualification, but that doesn’t mean I’m good at maths.
Adding という (というわけではない)
わけではない is often heard with the quoting phrase という. Used together, it softens the denial even further. The closest translation would be “that’s not to say…”
全く悪いというわけではない。
That’s not to say it’s altogether bad.
お金があれば必ず幸せになれるというわけではありません。
Having money doesn’t necessarily mean that you will be happy.
餃子が好きだけど、毎日食べたいというわけじゃないよ。
I like gyoza, but that’s not to say that I want to eat it every day.