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)’.
(The thing known as) Muscles need to be trained, or you lose them quickly.
(Things known as) Police need to protect the citizens from crime.
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, ~’.
You shouldn't say anything that could be taken as discriminatory. What I mean to say is, we’re all human.
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.
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.
What are side dishes?
I can't believe that person is 70. (70 years old, as I know it)
Dreams are so hard to give up. (Dreams as we know them)
Are laws actually necessary? (Laws as we know them)