Retail jargon used to describe a sales strategy in which a salesperson should constantly look for new prospects, pitch products or services to those prospects and complete the sale. Always be closing (ABC), as a strategy, requires the salesperson to be persistent, but also know when to cut losses and move on to another prospect.
The phrase was popularized in the 1992 film "Glengarry Glen Ross". In the film, an aggressive representative from the corporate office is brought in to motivate a group real estate salesman to sell more property or be fired if they fail. The representative opines on the "always be closing" mentality, a strategy which ultimately pushes the characters to be unscrupulous and dishonest in order to avoid being fired.