Strokes

Chinese characters are made up of strokes; each of the lines you see in a character is a stroke. Consider for a moment why they are called strokes....
Before pens and pencils, characters were written with brushes, thus the term stroke. There are eight basic strokes, and many combinations.

Note how each stroke has its own look, length, and direction.
Now look here for a more comprehensive listing of strokes.
When we write Chinese characters today with a pen, there is no way to vary the width of the strokes, but writing them correctly will have an impact on how your characters look. It is a good idea to keep the strokes in mind as you learn and write. Just as in English, good handwriting is valued highly by the Chinese.

Recommended Reading

https://www.clearchinese.com/chinese-writing/strokes.htm

https://en.wikipedia.org/wiki/Stroke_(CJK_character)

https://mandarin.about.com/od/characters/a/The-Importance-Of-Strokes-In-Chinese-Characters.htm