Onomatopoeia is the art of describing things or actions by imitating or creating sounds.
While in English and other European languages, they are mostly used to describe actual sounds, such as meow, woof, bang etc. Japanese utilizes a wide variety of onomatopoeia for all kinds of situations. It is very important to at least understand their meaning during conversation since they are often used, and if you want to go even further than that, using them yourself will make you sound more natural.
Japanese Onomatopoeia can be divided into five categories:
The first two groups contain expressions that are used to describe actual sounds.
Giseigo are only used for voice-related sounds (of animals or humans) such as わん (wan = woof), にゃん (nyan = meow) or うわーん！(uwaan = a child crying).
Giongo basically cover all the other sounds like ザーザー (zaa zaa = heavy rain) or めらめら (mera mera = suddenly bursting into flames).
Gitaigo, are used to describe states or conditions. These are expressions such as がたがた (gata gata = rattling/clattering) or むしむし (mushi mushi = hot and humid).
Giyougo are usually used for motions or movements (often related to travelling from one place to another). Among these, you will find expressions like うろうろ (uro uro = wandering aimlessly) and グータラ (guutara = not having enough will power to do anything), ごろごろ (goro goro = rolling around on the floor).
Gijougo, contains words that describe certain feelings and emotions like i.e. ウキウキ (uki uki = cheerful) or うっとり (uttori = being fascinated by something beautiful).
These are just some examples. There are thousands of onomatopoeia in the Japanese language used in countless situations. Using them, you can talk about the weather, temperature, food, sickness, character traits, shapes, accidents or even sports. They are therefore extremely convenient in daily life and not to be underestimated.