With the meaning‘not’

Prefixes (un-, in-, il-, ir-, and dis-) are often used to give adjectives (and some verbs and nouns) a negative meaning. Here are common examples:

happy unhappy like (v) dislike (v)
possible impossible legal illegal (= against the law)
correct incorrect regular irregular, e.g. irregular verbs

un- is used with many different words, e.g. unfriendly, unable, unemployed (= without a job), untidy (= not in order; in a mess)
im- is used before some words beginning with m or p, e.g. impolite (= rude), impatient (somebody who is impatient wants things to happen now; they cannot wait for things)
il- is used before some words beginning with 1, e.g. illegible (= cannot be read because the writing is very bad)
ir- is only used before some words beginning with r, e.g. irresponsible
dis- is used before some adjectives, e.g. dishonest, and a few verbs, e.g. dislike, disagree
in- is used before a limited number of words, e.g. invisible (= cannot be seen)

Note: A prefix does not normally change word stress, e.g. happv/unhappv: possible/ impossible. But the stress may change if you want to emphasise the negative or opposite:
A: Was he happy about the change?
B: No, he was very unhappy about it.

Verb prefixes: un- and dis-

These prefixes have two meanings: they can have a negative meaning (as above), but they can also mean ‘the opposite of an action’ or ‘to reverse an action’. This meaning is used with certain verbs.
I locked the door when I left, but I lost the key, so I couldn’t it unlock it when I got back.
I had to pack my suitcase (= put everything in it) very quickly, so when I unpacked (= took everything out) at the hotel, most of my clothes looked terrible.
The plane appeared in the sky, then suddenly disappeared behind a cloud.
In the morning you get dressed (= put on your clothes); when you go to bed you get undressed (= take off your clothes).

Other verb prefixes with specific meanings