What does `%{%v:lua.…%}` mean in an option?

Question

Currently trying to understand my neovim setup better.

So, looked into what a few classical options are set to.

:set statusline

yields

  statusline=%{%v:lua.require'heirline'.eval_statusline()%}

:help %tab was not very helpful.

So, could someone help me pick this apart? I found out %{%…%} is a lambda expression which gets re-evaluated as statusline expression.

v: would suggest we're entering visual mode, but that makes no sense in the context.

So, I feel completely noobish, here.