というのは is a phrase that combines the 格助詞（かくじょし） (case marking particle) と, the う - Verb 言（い）う, and the nominalizing particle combination のは. Together, they are usually translated as ‘the ~ known as ~ is ~’, or ‘the ~ called ~ is ~’. Sometimes, it can even simply be ‘~ means ~’. In essence,
というのは is just a way to turn an entire phrase into a noun, before explaining that noun further. This explanation of (A) will either be to give a definition of it, or to give a reason for it.
To use
というのは, attach it to the end of a phrase that you would like to indicate as being something that will be explained further ‘(A)’, and then follow it with your definition/reason, ‘(B)’.
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筋肉（きんにく）と言（い）うのは、鍛（きた）えないとすぐになくなる。
(The thing known as) Muscles need to be trained, or you lose them quickly.
警察（けいさつ）というのは市民（しみん）を犯罪（はんざい）から守（まも）らなければいけません。
(Things known as) Police need to protect the citizens from crime.
というのは is sometimes used at the beginning of a new sentence, when the speaker wants to add extra information about what has already been said. In these situations, the translation is similar to ‘what I mean to say is ~’, or ‘because of that, ~’.
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差別（さべつ）になることを言（い）ってはいけません。というのは、私達（わたしたち）はみんな同（おな）じ人間（にんげん）だからです。
You shouldn't say anything that could be taken as discriminatory. What I mean to say is, we’re all human.
あなたが言（い）った事（こと）を否定（ひてい）するつもりで言（い）っていません。というのは、私（わたし）はそれが間違（まちが）っていると思（おも）ったから言（い）ったのです。
I don't mean to disprove what you just said. What I mean is that, I said what I said because I personally thought that it was wrong.
というのは is regularly abbreviated as とは, or って. This can be a little bit hard to identify in casual speech, but the ‘(B) explains (A)’ concept will still almost always apply, so should be identified that way.
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おかずとはなんですか。
What are side dishes?
あの人（ひと）が70歳（さい）とは信（しん）じられない。
I can't believe that person is 70. (70 years old, as I know it)
夢（ゆめ）って簡単（かんたん）に諦（あきら）められないよね。
Dreams are so hard to give up. (Dreams as we know them)
法律（ほうりつ）ってほんとうに必要（ひつよう）なのか。
Are laws actually necessary? (Laws as we know them)