A revolving line of credit with a bank. Basically, bank says you have (to make up some numbers) revenues of $15,000,000/year and profits of $5,000,000, so we think it would be pretty safe to lend you up to say $1,000,000 that you can borrow whenever you want to run your business and you'll pay it down on some agreed upon interest rate (which is likely not fixed, but some percentage over prime rate[1]). Notably, you don't have to borrow, it's just available with pretty much no questions asked whenever you need it. Kind of like a credit card, but with a lower APR. It's very commonly used even for profitable businesses since cash inflows often don't match expenditure flows, especially if your company is growing.

For example, maybe you grew your company a lot this year and ran out of money in November, but a bunch of your customers will be paying you large sums when annual renewals renew in January. You can't just not pay employees in December, so you borrow for a month or two in order to smooth over payroll and other expenses and then pay it back in January.

Typically, a revolving line-of-credit is going to help with month-to-month expenses, but won't be enough to massively grow your business. For example, if you are a startup, a bank won't give you nearly as much money as a VC, but at the same time, you probably don't want to give up equity in your company every time you are temporarily behind on payroll. If you are a very large and established company, you'll likely have other, cheaper ways of getting money for your day-to-day business. For example, Commercial Paper is basically a short term bond issued by large well-known and creditworthy companies that need to smooth over payroll and other operational expenses. But by the time you are doing things like that, you probably have an entire corporate finance department handling these sorts of things. A revolving line-of-credit is much simpler.

[1] prime rate is the interest rate that big banks can borrow at which is typically lower than what you can borrow at. They'll borrow at rate Y and charge you rate Y+Z.

A revolver is a line of credit that companies use for financing. You can think of it almost like a credit card for a company — they make some general purpose agreement with a bank for $X of credit at y% interest, and can use that at their discretion. Companies generally raise debt through specific bond offerings, so this is just a quicker way for them to tap capital instead of going to market each time.

In case the other answers are not obvious enough, a term loan is borrowed once; you borrow $X, and the pay it back before the end of the term. A revolver for $X can be tapped and repaid many times, so long as the total borrowed amounts outstanding at any one time do not exceed $X.

In addition to the other explanations it might be interesting to know that high net worth individuals tend to make heavy use of these. They have extensive collateral that is often less than fully liquid. They can borrow a relatively small amount compared to their holdings and fund most of their day to day costs from their line of credit. Paying down the line can be done in a tax advantaged way. The math works out that they may pay low or even no taxes as a result.

A revolving credit line