Certificates of Deposits (CDs) are issued by banks. They're insured by the federal government against loss, up to a large enough number such that the typical freelancer or small consultancy would expect to never lose a penny of principal. Unfortunately, they're very poor for holding cash for freelancers/small businesses: 1) Interest rates are currently terrible virtually everywhere. 2) CDs have a duration, set in advance. 3 months, 6 months, 12 months, 5 years, etc etc. You can typically redeem them early, essentially at-will, but will pay a penalty to do so.
The option which you probably want to recommend to American freelancers/small businesses is called a "money market fund." It is a demand account, held at a financial institution, from the perspective of the customer. You can get money out of it essentially instantaneously. The money market fund invests in short-term debt, and historically they're very, very good about not losing principal. (One lost principal -- "broke the buck" -- as a result of a municipal bankruptcy in the 90s, and one other threatened to break the buck during the financial crisis.) The federal government treats money market funds as systemically important, so these days they're also explicitly federally backed (and implicitly backed by the fund sponsor).
MMF interest rates in the United States are currently also very low -- 0.5% yearly would be a pretty representative rate at the moment.
The best reasons for good cash management in the current interest rate environment are a) peace of mind, b) avoiding having to borrow money at credit card interest rates (typically the only credit source available to very small businesses in the US), and c) sometimes you can pre-pay vendors for things on terms which are scandalously good relative to short-term investment opportunities. For example, when I have a few thousand dollars burning a hole in my pocket, pre-paying a SaaS vendor often locks in a 10 to 20% APR-ified discount.