"I tell you, on the day of judgment people will give account for every careless word they speak, for by your words you will be justified, and by your words you will be condemned." Matthew 12:36-37
Fruit trees effortlessly produce fruit. It's what they do. In the same way, human beings effortlessly produce words. It's what we do, by nature. This is what's meant by the word careless here.
I think this verse is often misunderstood. In context, it doesn't mean that believers will be made to answer for every single careless, evil word they have ever spoken. In fact, it means quite the opposite. The words of interest for believers will not be careless, evil words, but careless, good words.
The verse means that everyone's unplanned, naturally-spoken (and long-forgotten) words will be examined, and no other evidence will be needed to determine if we are God's children or children of the devil. Children of God speak effortlessly for God; children of the devil speak effortlessly against him.
You who are yet in rebellion against God, your spontaneous words against him will be proof enough of your condemnation on the terrible Day when all your life is laid bare. Who would speak like that but a cosmic rebel? You who are regenerate, your spontaneous words for God will be proof enough of your justification. After all, who would speak like that but a forgiven sinner?
This is not a call to reckless speech. It's a call to examine how you talk when you can talk how you want. How do you speak in the privacy of your own home, in the break room, at lunch, with friends? When you are quite at your ease, what kinds of words find their way out of your mouth? These words are the truest judge of your soul condition; they are the fruit that prove what kind of tree you are, healthy or rotten.
Jesus spoke perfectly out of the majestic overflow of a perfect heart his entire life, and he did that for us. If you find yourself in a lost condition, know that Jesus lived for sinners just like you, and he drank God's judgment on the cross for sinners just like you. Run to him! Throw yourself entirely on his finished work, and you will be safe.