You may not be as familiar with the names of companies outside the United States, which might make you feel like the stocks and bonds they issue are risky. But if you invest in an international mutual fund or ETF (exchange-traded fund), you're increasing your portfolio's diversification by getting access to hundreds—sometimes thousands—of foreign securities.
Markets outside the United States don't always rise and fall at the same time as the domestic market, so owning pieces of both international and domestic securities can level out some of the volatility in your portfolio. This can spread out your portfolio's risk more than if you owned just domestic securities.