Hello Zeynep,
The word you mean is "daijoubu".
or 「だいじょうぶ」 and in Kanji・・・「大丈夫」
It means:
OK
Alright
That's okay
It's okay
...and so on.
In it's most basic form it is equal to:
" Okay, OK, or alright"
...in English.
Like if someone bumps into you and says, "I'm sorry."
And you say, "That's OK."
In Japanese:
Person: すみません。
You: 大丈夫です。
Hope that helps.
Have fun using it!