Lockout/tagout is an 'umbrella' term that describes a set of safety practices and procedures that are intended to reduce the risk of workers inadvertently using tools or equipment or materials that have been determined to be unsafe or potentially unsafe.
An example of 'lockout' would be physically securing broken or unsafe tools in some way or in some place so that they could not be accessed and used by someone who may be unaware of the potential danger of doing so.
An example of 'tagout' would be a clear and unavoidable label affixed to a piece of equipment that describes the fault found with it, together with a warning not to use it.
Standardized lockout/tagout procedures are a mandatory part of workplace safety regulations in most countries. Please familiarize yourself with your local legislation and with the specific lockout/tagout practices that apply in your workplace.