What does a red X made with tape on a door mean?

A Red X typically indicates "stop, do not enter". Whether it is a prefabricated sign, or drawn on cardboard in crayon, or made with red tape, the meaning remains the same, especially if posted on a door.

During times of plague, a red X was to serve as a warning that occupants of the house were sick, so do not enter.

In disaster search and rescue, Federal Emergency Management Agency (FEMA) Search and Rescue Teams use a Red X on housing/structures (with Date) to indicate "structure unsafe to enter" and people must have permission, in writing, to enter to make repairs. It does not mean the structure is condemned, just unsafe in its current condition. Structures would also be marked to show Searched; Deaths/Bodies Recovered; Deaths/Bodies not recovered yet; Date searched...etc. Each "quadrant" of the X holds this specific information, ALWAYS with specific info put into one specific portion of the X. See this page for an example picture.