Backquotes allow selective element evaluation:
`(,(format-time-string "%Y") . ?y)
, indicates an element to be evaluated, so
,(format-time-string "%Y") will evaluate to the current year (e.g.,
?y will remain unevaluted.
org-tag-alist to create a tag for the current year, mapped to the
(setq org-tag-alist `((,(format-time-string "%Y") . ?y)))
Using backquotes you can even splice lists into alists using
,@ notation, see doc for details.
I have used elisp for years and never encountered backquotes! Hoping this will help someone find it when web searching. Credit to this SO answer I stumbled upon while searching for a related issue.