Apologies in advance, as we are going to be a wee bit repetitive here. Repetitive because Japanese has this amazingly amazing capacity to form words through the simple process of repetition, or what linguists call reduplication.
The Japanese term for this craft is 畳語 (jōgo), a compound of the characters for 畳む (tatamu, fold) and 語 (go, word). The idea is that there are two layers of the same piece of structure folded over one another, just like the straw layers of a tatami mat. So, for the sake of simplicity let's call them tatami words — and unfold them a little bit.
Perhaps the most frequent — and definitely best-known — type of tatami words are onomatopoetic expressions. It's きらきら (kirakira) for things that twinkle, かりかり (karikari) for things that crunch, ぺらぺら (perapera) for speaking a foreign tongue with ease, and べらべら (berabera) for doing the same with one's own tongue. These are just a few of an almost uncountable number of such expressions. In fact, there are so many that one sometimes wonders whether there is any repeat combination of two kana left that does not claim some onomatopoetic meaning.
With your current subscription plan you can comment on stories. However, before writing your first comment, please create a display name in the Profile section of your subscriber account page.