Diversification
Not putting all your eggs in one basket. Spreading money across many companies, countries and kinds of asset makes any single disaster hurt far less.
One company can fail
Even famous companies collapse. If one is 50% of your money, its bad year is your bad year. Spread over hundreds, the same collapse is a blip.
Spread in more than one way
Many companies is a start. Also spread across countries (not only your own), sectors (not only tech), and kinds of asset (shares, bonds, some cash). Things that don't fall together soften each other's falls.
Check what you really own
Three different funds can all be mostly the same big US companies. Allown's X-ray looks inside your funds and adds everything up, so you see your real spread by company, country and sector.