Verified answer

Low key is a more used term. Meaning small scale, subdued, quiet.

High key is therefore the opposite. Big scale, loud, expensive.

A high-key image consists primarily of light tones, without dark shadows.[1] A photograph or painting so composed features a diminished tonal range of primarily whites and light grays. [2] High key as a term used in describing paintings or photographs is related to but not the same as high-key lighting in cinema or photography.[3]

https://en.wikipedia.org/wiki/High_key

having or producing light tones only with little contrast —used of a photographic print or subject or of the lighting of a photographic subject

https://www.merriam-webster.com/dictionary/high-ke...

Having a predominance of light or bright tones.

‘The colors are often high-key but eschew quick optical effects for slower vibrant resonances.’

‘The 13 paintings on view included work from the mid '60s to the early '90s, but with their high-key color and crisp, consistent technique, all the paintings seemed freshly finished.’

‘His pleasant and undemanding style was marked by brilliant high-keyed colouring and vigorous brushwork, representing a kind of conservative version of Impressionism.’

https://en.oxforddictionaries.com/definition/us/hi...

(of a photograph) having chiefly light tones, usually with little tonal contrast (distinguished from low-key

http://www.dictionary.com/browse/high-key

(of a photograph, painting, etc) having a predominance of light grey tones or light colours

https://www.collinsdictionary.com/dictionary/engli...

https://www.infoplease.com/dictionary/high+key

It doesn't mean anything. If the writer is Japanese, they often use phrases that they think sound good that are not normally used by native speakers of English. From the context, I would assume he means what we call a "big deal" or very important.