A Taxable Investment Account is an account that can be opened at any brokerage to invest in stocks, bonds, or funds. After fully contributing to all tax advantaged accounts (401k, HSA, Roth IRA), leftover savings should be invested in a taxable account.
It's important to invest your savings, ideally in low cost index funds. By keeping it in cash, your savings will lose value overtime from inflation.
Brokerages where you can open an investment account include: