DIY stands for Do-It-Yourself.
Today, more than ever, because of the internet, more and more people are DIY'ing it. This includes everything from investing, to building electronics and circuits.
A generation ago, investors had to pay huge sums of money on commissions to buy any security. $150+ on a single stock buy order was the norm.
The information individuals were given was limited to what they were told by their broker. I vivedly remember my father asking his broker about certain stocks, if they are "good" or "bad stocks". It is laughable now, as it is certain his broker had no clue.
Along came the internet and changed all that. We now have access to low cost brokers, and tons of tools around the internet, all geared to help them invest.
Unfortunately these tools were made with one thing in mind. Now that commissions are radically lower, brokers need to get individuals to trade much more. The tools brokers have are mostly active trading tools. Charting, real time quotes, and derivatives. They do not help the DIY investor, just the active trader.
So in a nutshell DIY Investing has been widely overlooked. What we have today is just a lot of DIY Active Trading. Which pretty much explains why individuals perform so poorly
Oh and it should be mentioned, you can give your money to a professional advisor, who will charge a fee, for what you can do yourself for free.