Primary keyword is nothing but the phrase for which you are likely to rank for.
Phrase - which a user enters in the search Engine to find something.
And when it comes to Primary keyword of website, it is nothing but the website niche.
You might have already observed, the blogging niche folks domain get includes like blogger, blogging, seo, wp, wordpress etc.
Same thing goes with tech, travel, and food as well.
Infact there are some website which specifically works on a specific product like iphoneblablabla.com, samsungblablabla.com. Anything.
Though is is not compulsory at all, you might find several blogs have reach the heights without the primary keyword.
Few indians too like shoutmeloud.com and labnol.org are two major ones.