Contractions are words that have been formed by combining two words together. When we bump the two words together, some letters are removed. An apostrophe goes into the place where the letters came out.
The contraction means exactly the same as the two words. Look at this example:
I can not read this small lettering. I can’t read this small lettering.
It is fine to use contractions in casual writing and casual speech. However, writers should avoid using contractions in formal writing. Contractions should not be written in school essay assignments, letters to employers, resumes, or other forms of formal writing.