When Bill Gates dropped out of college to co-found Microsoft, he wasn't thinking about becoming a billionaire or running a company that's now valued at more than $3 trillion.

Gates, then 20 years old, had a much more "boring" definition of success, he tells CNBC Make It: "Back then, it was just: Is my code really good? Does it work? And can this company show the world that these microcomputers are big?"

At the time, in 1976, computer obsessives like Gates and co-founder Paul Allen were considered "hobbyists" — yet they fervently believed that a technological revolution was imminent. "It was the magic of software. And I was willing to focus my life, in my 20s, just on software, just on the one job," says Gates.

Specifically, that job was creating high-quality software that could make the general public actually embrace the personal computer. "Our phrase was 'a personal computer on every desk and in every home,' which sounds boring today, but back then [it] was completely crazy," says Gates, referencing the mission statement he and Allen often repeated to Microsoft's early employees.

That intense focus on creating the best product possible didn't mean Gates wasn't aware that there was also money to be made — in fact, he insisted upon it from the beginning. In his famous "Open Letter to Hobbyists" in 1976, Gates wrote that users needed to pay fair prices to use software so that developers, like himself, were compensated well enough to ensure they could work on creating the new, high-quality software the industry needed to grow.

Gates' singular mindset — "It was all Microsoft, all the time in my 20s ... my view of success was very much Microsoft-centric," he says — helped push the company to the forefront of the computer age, making him one of the world's wealthiest people in the process. His current net worth is $128 billion, Forbes estimates.

Today, Gates enjoys "being polymathic and learning lots of things," spending his free time reading about topics like biology, physics and climate science, he says. Those subjects are the focus of an upcoming Netflix docuseries called "What's Next? The Future With Bill Gates," set to premiere on September 18.

How Gates defines success today

Gates' personal definition of success has "definitely evolved" from his younger days, he says.

In part, Gates regrets his single-minded focus, which kept him — and his employees — from enjoying a sense of work-life balance. Now, he advises everyone to "take a break when you need to," he told students at Northern Arizona University's commencement ceremony last year.

"I don't work as hard [now]," Gates recently told Make It. "In my 20s, I didn't believe in weekends and vacations. So, that was kind of out of control, how I pushed myself."

Today, the 68-year-old's idea of success revolves around a different question, he says: "Am I adding net value [to the world]?" He's pledged to give away "virtually all" of his massive fortune — within the next 20 years, he's said — through initiatives like the Bill & Melinda Gates Foundation and his clean-energy investment group, Breakthrough Energy.

"Now, I can define my success in terms of empowering other people [by] sharing what I did wrong, what I did right and providing my resources to things like malaria or climate change," says Gates.

He's looking forward to continuing that work over the next few decades, if his health allows, he adds: "I'm just super lucky to be in a different phase [of my life], but still able to feel like [I'm] making a difference."

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