Does lockdown mean lockout? What's the difference between lockout and lockdown?

Lockdown

An emergency impacting a school campus that prevents the safe evacuation of a school building and requires steps to shelter students and staff from danger. Lockdown procedures may also be used to ensure the safety of students when an incident occurs in the vicinity of a school.

Lockout

It is used to secure the school or building from a potential threat taking place outside, such as law enforcement activity in the neighborhood. During a lockout, access to the building is restricted, but there may be controlled limited movement.

A lockout locks people out.

A lockdown locks some people in and some people out. If you're in, you can't get out. If you're out, you can't get in.

The term "lockdown", in it current most common sense, derives from its use in prisons, where, during a "lockdown", prisoners are confined to their cells rather than being allowed to mingle in common areas.

Wikipedia:

In its most common usage in corrections units, the term lockdown can be defined as a course of action to control the movement of inmates. Confining all prisoners, except workers, to their cells until the end of the day is an example of a "lockdown period" in a corrections schedule. However a "full lockdown" is used when all prisoners are locked in their cells to prevent prison riots or unrest from spreading or during an emergency.

The meaning of the term has been broadened to encompass, eg, the situation where a school may have its doors locked and students confined to classrooms during an "active shooter" event or some such.

The term is used somewhat figuratively when describing a "shelter at home" order, since doors are not locked (in the US, at least) and limited entry/exit is permitted.