adverb
Describes a short, sharp mechanical sound, like a switch, lock, or lid closing neatly.
スイッチをカチッと入れる。
I flip the switch with a click.
ふたがカチッと閉まった。
The lid snapped shut with a click.
adverb, noun or participle which takes 'suru'
Describes something fitting or being secured exactly in place, often with a sense of neatness or precision. Can be used adverbially or with する.
部品をカチッとはめ込む。
I fit the part firmly into place.
引き出しをカチッと閉める。
I shut the drawer tightly.