A taxable brokerage account is a regular investment account, opened at a custodian, that holds stocks, bonds, index funds, and ETFs — without any of the special rules that apply to retirement accounts. It's the most flexible account type, and also the least tax-favored.
No contribution limits
Unlike a 401(k) or IRA, there's no annual cap on how much you can deposit and invest. You can put in $500 or $500,000 in a single year.
No special tax treatment
Dividends and capital gains are taxable in the year you receive or realize them. Hold an investment over a year before selling to qualify for the lower long-term capital gains rate; sell sooner and the gain is taxed as short-term, at your ordinary income rate.
Full liquidity
Withdraw money anytime, for any reason, with no early-withdrawal penalty. That's the trade-off for giving up the tax advantages a 401(k) or IRA offers.
When it's useful
Once tax-advantaged space (401(k), IRA, HSA) is maxed out for the year, or for a goal that has no tax-advantaged account option — there's no "brokerage IRA" equivalent for a house down payment five years out. See the buckets of money, by tax treatment for how it compares to retirement accounts.