robots.txt is a plain-text file at a site's root that tells a crawler what it may fetch. It follows RFC 9309, a real internet standard, and well-behaved crawlers are expected to obey it, though nothing forces a bot to.
It answers a different question than llms.txt: robots.txt is a rule about
access, not a curated summary of what a site is. Birta's own robots.txt
points crawlers at both /llms.txt and /llms-full.txt.
For how the two files differ and what each is actually for, see what is an llms.txt file, a real one explained line by line.