Klok means 'clever', 'intelligent', or 'wise'. It might be a familiar adjective to you if you know other Scandinavian or Germanic languages: it has its roots in Old Norse (klókr) and corresponding words exist today in Norwegian (klok) and German (klug), for example.
You can hear how klok sounds in the clip below:
Swedish has other words for 'clever', including intelligent and smart, but klok has a strong connotation of taking logical, wise decisions and generally showing good judgment, as opposed to, for example, an exceptional talent for memorization or quick-thinking.
A person or group of people can be klok, for example min kollega är väldigt klok (my colleague is very wise) or regeringen har varit klok (the government has been clever), and an object or concept can also be klok, for example kloka ord (wise words), ett klokt förslag (a wise suggestion), or en klok idé (a clever idea).
And from around the 15th century up until the 20th, the terms klok gumma and klok gubbe (literally 'wisewoman' or 'wise old man') were used to describe folk healers (also known as 'cunning folk' in English). These were generally older members of communities who practised folk magic and worked as healers. Some of them faced punishment and prison terms for their actions, including Gotland klok gumma Brita Biörn, who said she had learned her skills on a visit to the underworld.
But back to the word klok as we use it today, and it's worth knowing that something strange happens when you negate it.
The phrase inte klok literally means “not wise/clever”, but when you hear a sentence like “du är inte klok!” it would usually translate into English as “you're crazy!” This might be confusing the first time you hear it, but makes a lot of sense if you think of klok as relating to good judgment rather than intelligence per se.