In Korean, when you are talking about the definition of something or when you want to express what you think is the definition or the nature of something, you can use the expression -(이)라는 것은. It can be followed by various types of sentence endings, including -라고 생각해요 (I think that...), -인 것 같아요 (It seems that...), etc.

-(이)라는 것은
-(이)라는 것은 is a shortened form of  -(이)라고 하는 것은. The structure -(이)라고 하다 means “to say …”,   and 것 [geot] means “thing” or “fact”, so together, -(이)라고 하는 것 means “the thing that is called” or “what they call …”.

Examples
부자라는 것은
[bu-ja-ra-neun geo-seun]
= What they call “부자” is ...
= A rich man is …
= (I think that) A rich man is ...

자유라는 것은
[ja-yu-ra-neun geo-seun]
= What they call “자유" is...
= Freedom is …
= (I think that) Freedom is …

우정이라는 것은
[u-jeong-i-ra-neun geo-seon]
= What they call “friendship” is...
= Friendship is...

사람의 마음이라는 것은
[sa-ra-mui ma-eu-mi-ra-neun geo-seun]
= I think a person’s mind/heart is...

More Examples
사랑이라는 것은
[sa-rang-i-ra-neun geo-seun]
= Love is...

삶이라는 것은
[sal-mi-ra-neun geo-seun]
= Life is...

꿈이라는 것은
[kku-mi-ra-neun geo-seun]
= Dream is...

Shortening -(이)라는 것은 to -(이)란
To make it even more convenient to say, people often shorten -(이)라는 것은 to -(이)란.

Ex)
사랑이라는 것은 → 사랑이란
삶이라는 것은 → 삶이란

Sample Sentences
1. 사랑이란 무엇일까요?
[sa-rang-i-ran mu-eo-sil-kka-yo?]
= What is love?

2. 사랑이란 쉽지 않아요.
[sa-rang-i-ran swip-ji a-na-yo.]
= Love is not easy.

3. 자유라는 것은 (자유란), 아무거나 마음대로 하는 것이 아니에요.
[ja-yu-ra-neun geo-seun, a-mu-geo-na ma-eum-dae-ro ha-neun geo-si a-ni-e-yo.]
= Freedom doesn’t mean doing whatever (you want to do) in any way you like.

4. 진정한 친구란, 어려울 때 도와 주는 친구예요.
[jin-jeong-han chin-gu-ran, eo-ryeo-ul ttae do-wa ju-neun chin-gu-ye-yo.]
= A true friend is a friend who helps (you) when things are difficult.