The fixed share count is the source of everything else. Because a closed-end fund does not create or redeem shares after its offering, the only way to buy is from another investor and the only way to sell is to another investor. Supply and demand for the shares themselves therefore set the price, and that price is not tied to the value of the fund's holdings. Net asset value, the fund's assets minus its liabilities divided by shares outstanding, is still calculated and published, but it is a reference point rather than the transaction price.
Premiums and discounts are the visible consequence. A closed-end fund's market price can sit above its net asset value, called trading at a premium, or below it, called trading at a discount. Discounts are the more common state, and they can persist for long stretches, which means an investor can buy a dollar of underlying assets for less than a dollar, or, less happily, sell for less than the holdings are worth. The gap is a feature of the closed-end structure, not a mispricing to be assumed away, and it introduces a second source of return and risk on top of the portfolio itself. An open-end mutual fund has no equivalent, because it transacts at net asset value; an exchange-traded fund keeps its price close to net asset value through a creation-and-redemption mechanism that a closed-end fund lacks.
Not facing redemptions lets a closed-end fund do things an open-end fund cannot. Because it never has to sell holdings to meet withdrawals, a closed-end fund can invest in illiquid or thinly traded assets, such as private loans, municipal bonds in specific niches, or emerging-market debt, without the risk that a wave of redemptions forces a fire sale. Many closed-end funds also use leverage, borrowing to buy more assets than the shareholders' capital alone would support, which raises income in good markets and magnifies losses in bad ones. These are the reasons the structure survives mainly for income strategies rather than for broad stock-market exposure.
The liquidity you have is exchange liquidity, which is not the same as the portfolio's. You can sell a closed-end fund any time the market is open, but only at whatever another investor will pay, which during stress can be a wide discount. That is a different kind of liquidity from an open-end fund's daily redemption at net asset value, and confusing the two is the main way a closed-end fund surprises an investor who bought it for its yield.
One important exception to everything above: the interval fund. The description on this page is of the exchange-listed closed-end fund, which is what the term usually means, and the SEC hedges accordingly, saying that "many closed-end funds generally sell all of their shares in a public offering, then trade on an exchange." An interval fund is legally a closed-end company too, but it continuously offers new shares at a price based on net asset value and buys shares back from holders at set intervals instead of listing them, so it has no market price and therefore no premium or discount. Where a fund transacts at net asset value with the fund rather than at a market price on an exchange, the premium-and-discount analysis on this page does not apply to it.