If you want to invest in Apple, Amazon, Tesla, Pets.com, etc. but don't have the money for a whole share you can get a Robinhood account and buy fractional shares. They let you buy a sliver of a share for as low as $1.
If you don't have a lot of knowledge on the subject or just don't want to have to study to predict what's coming then buying ETFs for sectors, indicies, or broad markets might be the better way to go. Since they are the equity from different companies bundled into one instrument they allow you to diversify automatically.
I didn't know Stash let you go as low as a penny. They're being kind of literal.