The eggs and baskets idea
One stock can crash. A basket of 500 companies almost never all crash at once. Spreading out reduces the damage from any single bad pick.
How beginners diversify
The most common tool is a low-cost index fund. A single purchase can give you tiny slices of hundreds of companies at once — instant diversification.
It is not a magic shield
Diversification lowers the risk of one bad pick wrecking you. It does not remove the risk of the overall market going down for a while. That is why time horizon still matters.
Example
Instead of $1,000 in one tech stock, $1,000 in an index fund gives you a sliver of hundreds of companies across many industries.