Diversification is the only free lunch in investing. You invest in many different things. If one fails, the others gain.
The egg example
I have 1000 euros. If I put it ALL in one stock and that stock crashes, I lose everything.
If I diversify (200 in 5 different stocks), if one crashes, I lose only 200.
How to diversify
Stocks from different sectors: tech, pharma, energy, banking.
Different countries: Spain, USA, Japan.
Different asset types: stocks, bonds, gold, real estate.
Funds do it automatically
An index fund has hundreds of stocks. It's already diversified. It's the easiest way.
The 70/30 rule
70% in stocks (high risk, high reward).
30% in bonds (low risk, low reward).
When the stock market drops, bonds protect you. When it rises, stocks gain big.
Geographic diversification
If all your investments are in Spain and Spain enters crisis, you lose everything.
If you have money in USA, Europe, Japan, if one fails, the others gain.
Risk still exists
Diversifying doesn't make you immune to risk. If the entire global economy collapses, your diversification doesn't help. But that's rare.
The truth
A well-diversified expert earns less than an expert with one winning bet. But loses less when the bet fails.
For most people, diversifying is the right strategy.