These posts discuss this in more detail:
ETF: Concentration risk in ETFs
Concentration risk
Concentration risk refers to the risk we take when we invest too much money as a percentage of our overall portfolio into a single company or sector. Should something go wrong with the company or sector, a higher percentage of our portfolio underperforms, dragging down the performance of all our investments combined. Concentration risk is the finance term for the idea that you shouldn’t put all your eggs in one basket. A diversified portfolio will help you avoid concentration risk.