Things, not strings. If you haven’t heard this before, it comes from a famous Google blog post that announced the Knowledge Graph.

The announcement’s 11th anniversary is only a month away, yet many still struggle to understand what "things, not strings" really means for SEO.

The quote is an attempt to convey that Google understands things and designs-tab-open is no longer a simple keyword detection algorithm.

In May 2012, one could argue that entity SEO was born. Google’s machine learning, aided by semi-structured and structured knowledge bases, could understand the meaning behind a keyword.

The ambiguous nature of language finally had a long-term solution.

So if entities have been important for Google for over a decade, why are SEOs still confused about entities?

Good question. I see four reasons:
Entity SEO as a term has not been used widely enough for SEOs to become comfortable with its definition and therefore incorporate it into their vocabulary.
Optimizing for entities greatly overlaps with the old keyword-focused optimization methods. As a result, entities get conflated with keywords. On top of this, it was not clear how entities played a role in SEO, and the word "entities" is sometimes interchangeable with "topics" when Google speaks on the subject.
Understanding entities is a boring task. If you want deep knowledge of entities, you’ll need to read some Google patents and know the basics of machine learning. Entity SEO is a far more scientific approach to SEO – and science just isn’t for everyone.