Doki doki (どきどき) is the sound of a pounding heart, used for feeling nervous, excited or in love.
Put your hand on your chest before an exam, a first date or a jump scare. That fast thump-thump is どきどき — and in Japanese the sound itself has become the word for the feeling.
What it means
どきどき is the sound of a heart beating fast. People use it for the feeling that makes the heart do that: being nervous, excited, in love, or a little scared. English needs a different word for each. Japanese uses one, and the situation tells you which.
How to use it
Add します and it works like a verb: どきどきします (“my heart pounds”, “I get nervous”), どきどきしています (“it is pounding right now”), どきどきしました (“it was pounding”). That one pattern is enough for almost everything.
あした、はっぴょうが あります。 どきどきします。
I have a presentation (はっぴょう) tomorrow. I'm so nervous.
だいじょうぶ。がんばって！
You'll be fine. Good luck!
Close cousins
わくわく is happy excitement with no fear in it: the night before a trip. はらはら is the worry of watching someone else in danger: a friend on a tightrope. どきっと is one single thump: the jolt when someone suddenly says your name. どきどき is the one that keeps going.
In manga and anime
As a sound effect it is usually written in katakana, ドキドキ, floating beside a character whose heart is racing. A single ドキッ marks the moment of surprise. Once you can read katakana you will start to see it on almost every page of a romance manga.
Can you use it?
Yes, and it sounds natural and friendly, not childish. Adults say it before interviews and speeches all the time. It is one of the easiest ways for a beginner to talk about feelings without any difficult grammar.