The legal separation of client securities from a broker’s or fund company’s own assets, so they are protected if the firm fails.
Securities in a brokerage account and the assets of an ETF or mutual fund belong to investors, not to the firm. In the US, SIPC additionally covers up to $500,000 per customer (including $250,000 in cash) if assets are missing after a broker failure. Bank deposits are different: they are claims on the bank, protected by deposit insurance (FDIC up to $250,000 in the US, €100,000 in the EU).