The three characteristics (anicca, dukkha, anatta)

3 common properties

Everything in the universe have three common properties: anicca, dukkha, and anatta. People, animals, things, towns, mountains, planets, stars, joy, money, knowledge, memories, everything you may imagine, everything you can't imagine. Every possible thing is anicca, dukkha, anatta. These three words are Pali words (Pali was the language spoken by the Buddha). Let's together examine what they mean...

anicca

"anicca" (pronunciation: "anicha") means "unpermanent". Nothing possibly lasts forever without ending sometime, one day. When you enjoy eating a good chocolate cake, even if your pleasure lasts long, it can't last forever. It's the same for everything.
Some things do not last long, for instance: butterflies, flowers, batteries, handkerchiefs... Other things last long: houses, mountains, rivers... Nevertheless, even these things will come inevitably to an end.
That's why, if one is reasonable, one tries to know reality better and better to be less and less involved in clinging. That's the aim of practising dhamma (buddhist practise). More we cling, more we suffer. For instance, if you cling to a toy or a book, you will be desapointed or stressed if you lose it. If you cling to a place, you'll be sad when moving from there. If you cling to a principle, or an idea, you'll be frustrated if one goes the other way. If you do not cling to anything, you'll never be desapointed or irritated.

dukkha

"dukkha" (pronunciation: "dook-kha") means "suffering" or "unsatisfaction". Nothing can be wholy satifying, completely pleasant. Everything in life includes a part of suffering, even best ones, because of limits. Everything we can do is always unperfect. When a moment is pleasant, when all is well, it never lasts long. You enjoy something now because you were in pain before.
Even if the whole world was yours, if you could do everything you want, you will surely find something wrong.
The only way to always feel happy is to possess nothing, not to be involved in entertainments, and to be satisfied with very few things in life. Even so, you may be sick, injured, or get other trouble.
A wise person can see clearly that suffering, or insatisfaction, is everywhere. That's why one stays more quietly and knows how to feel good with few things.

anatta

"anatta" (pronunciation: "anat-ta") means "does not exist by itself". Every existing thing is only the combination of several things, is divided in several elements. When you admire a nice phone or a nice bike, this thing does not exist by itself: it's just a combination of small pieces. It's the same for everything.
Even you do not exist by yourself: you are the result of a natural process and your whole body is a set of particles, like pieces of a puzzle: there are eyes, teeth, hair, bones, flesh, blood, grease, and so on.
When one knows properly how to face the reality, one gets less involved in desires, because one understands that all existing things are nothing but succession of causes and results. One knows that nobody can force something to happen just as he wishes, because every happening things never depends of one single thing. In the other way, one cannot everytime avoid what he doesn't want.
More you long for things, more you get worry, because things almost never happen like we want. Better you get satisfied with situations as they happen, better you feel.