Asset allocation is the recipe for your portfolio: what share is in stocks, what share in bonds, and what share in cash or similar. Stocks offer higher long-run growth with big swings; bonds offer steadier but lower returns; cash offers stability and nothing else. The mix you choose matters more for long-term results than which individual funds or stocks you pick.
A common rule of thumb is to hold your age in bonds, or a more aggressive version, your age minus 20. A 30-year-old might hold 80% to 90% stocks; a 60-year-old closer to 50% to 60%. Target-date funds do this automatically, shifting from stocks toward bonds as the target year approaches.
The allocation should match when you need the money, not your feelings about the market. Money for a house down payment in two years belongs in cash or short-term bonds, however tempting stocks look. Retirement savings for someone in their thirties can sit almost entirely in stocks because a downturn has decades to recover.
Employer stock deserves a specific line in the allocation. RSUs and ESPP shares accumulate quietly, and it is common to find 30% or 40% of net worth in one company without having decided that. Most planners suggest keeping any single stock under 10%.
The common mistake is setting an allocation and never revisiting it, so that a long bull market leaves an intended 70/30 portfolio at 85/15 right before a downturn.