A lockout is a refusal by the employer to provide opportunities to work; it (sometimes literally) locks out employees and prohibits them from doing their jobs (and being paid). The NLRB views lockouts and an unfair labor practice only when the employer acts for a prohibited purpose. Lockouts are not widely used today; employers are usually reluctant to cease operations when employees are willing to continue working.
Lockout is the management decision to put pressure on union members by closing the firm.