Well, it refers to an asset that is not readily sold for cash. This may be because there is a small market for that item, and so there is not an instant demand for the product.
If you trade some specialist product such as fancy diamonds for instance, then when you want to sell a sparkling yellow diamond for cash, then it is not something that happens instantly: there are specialist collectors, and they would want to value the stone and only then you would have an offer. Similarly with financial products and instruments that have a very limited secondary market.
Compare this to more mainstream assets where there is a much wider and more ready market, and those assets in contrast are liquid as they are easily sold for cash.
Legal rate of interest