What Does De-listing Mean?
The removal of a listed security from the exchange on which it trades. Stock is removed from an exchange because the company for which the stock is issued, whether voluntarily or involuntarily, is not in compliance with the listing requirements of the exchange.
Investopedia explains
The reasons for delisting include violating regulations and/or failing to meet financial specifications set out by the stock exchange. Companies that are delisted are not necessarily bankrupt, and may continue trading over the counter.
In order for a stock to be traded on an exchange, the company that issues the stock must meet the listing requirements set out by the exchange. Listing requirements include minimum share prices, certain financial ratios, minimum sales levels, and so on. If listing requirements are not met by a company, the exchange that lists the company's stock will probably issue a warning of non-compliance to the company. If the company's failure to meet listing requirements continues, the exchange may delist the company's stock.