What do double curly braces mean in systemd scripts?
Solution 1:
The curly braces mean nothing to systemd. The other question was either using them as a placeholder for a variable name or using some kind of template to generate systemd files.
Solution 2:
In the linked example, {{ venv_home }} is a general placeholder for the location of the virtual environment in question (venv). For example, this could be /home/pi/yourproject. The double-curly-brace notation appears a bit flamboyant, which may have added to the confusion.