for example this investment can be in your own business that you start and by expanding it you make an steady income over time.
I also believe that having your own business is the key to have a steady income. But before engaging into any kind of business, make sure that you know your business and it is your passion because you will not be able to sustain it if you don't like it in the first place.
I don't agree with having to start a business revolving around your passion, just do it around something that will make you money, you can always get passionate about new stuff, I'd argue.