An onomatopoeia is a word that mimics a sound - for example "woof", "bang", or "moo". While they are popular in English (to the point where some onomatopoeias are no longer commonly recognised as such, like the word "bleat" to mimic the sound a sheep makes), they are absolutely rife in Japanese. In fact, unlike English, Japanese has two kinds of onomatopoeia: Giongo (擬音語) are words that directly imitate sounds (there's a subgroup of these just for animal and human sounds, called Giseigo (擬声語)), and Gitaigo (擬態語) are words that express emotions, actions, or conditions. Both Giongo and Gitaigo always take the form of a doubled word - for example, a dog barking is "wan wan" (ワンワン).
Giongo are like onomatopoeia in English and are pretty self-explanatory. However, Gitaigo, which are symbolic or mimetic words, are more abstract and interesting for foreign speakers. Japanese people litter these throughout their conversation without giving them a second thought, but for non-native learners of Japanese they can often be confusing, difficult to translate, and a pain to learn.
Here are some examples of Gitaigo and what they mean:
giri giri (ギリギリ) - "just barely", "by the skin of your teeth"
bero bero (べろべろ) - "drunk"
pika pika (ピカピカ) - "sparkling", "shining brightly"
uzu uzu (ウズウズ) - "eager", "raring", "itching to do something"
uki uki (ウキウキ) - "happy", "in a good mood"
hiso hiso (ヒソヒソ) - "in a whisper"
nita nita (ニタニタ) - "smirking", "grinning"
Some of these may seem very random, especially since they are classified as onomatopoeia. But if you read closer, some of them reveal their mimetic nature. For example, hiso hiso mimics the sounds of a whisper; it's almost impossible to form the sounds for uki uki or nita nita without your mouth curling into a smile; and bero bero perhaps mimics an inebriated person slurring their words.