“I don't buy into the idea that you have to go to college to get a good job.”
You are talking with your brother about his son (your nephew), who's in high school but not getting very good grades. Your brother is worried about him not being able to get into college and get a good job later in life. Although most people think it's important to go to college, you don't agree. You say this.
I don't buy into the idea that you have to go to college to get a good job.
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To "buy into" an idea means to believe in that idea.
People usually use "buy into" in a negative sense - "I don't buy into that." If you do agree with an idea, you use another phrase like "believe in":
I believe in the idea of getting a good education.
You can use this phrase to turn a clause (which is like a sentence) into a noun so that you can talk about that idea. For example, if you take an idea like "Buying a house is better than renting," you can ask questions about that idea:
What do you think of the idea that buying a house is better than renting. Do you think that's true?
This is a common phrase for expressing what must be done to get something that you want. Its good for casual and conversational situations. There are some famous phrases that use this structure:
You have to spend money to make money.
You have to give respect to get respect.
To make this phrase more formal, change "have to" to "must" and insert "in order to":
You must take three semesters of mathematics in order to graduate.
A "good job" is one that pays a lot of money and is stable.