An ignorant person doesn't know or understand something that they should.
When we talk about someone being ignorant, it usually means they lack information or knowledge about something. We might say someone's ignorant of politics if they don't follow current events or understand how government works. It could also mean they're not aware of cultural differences or nuances, like not realizing why a certain expression is considered rude in another country. In some cases, people are accused of being ignorant when their opinions or actions seem to come from a lack of understanding rather than a deliberate choice.
How common is "ignorant"?
Word ignorant is considered uncommon in modern English. It has a balanced usage among all categories: speech, fiction, newspapers and academic texts.
Definitions
adjective
Unknowledgeable or uneducated; characterized by ignorance.
Not knowing (a fact or facts), unaware (of something).
Example: 1851, Walt Whitman, “Art and Artists” in Emory Holloway (editor), The Uncollected Poetry and Prose of Walt Whitman, Garden City, NY: Doubleday, Page & Co., 1921, Volume 1, p. 242,
[…] perhaps it is sometimes the case that the greatest artists live and die, the world and themselves alike ignorant what they possess.
(slang) Ill-mannered, crude.
Example: His manner was at best off-hand, at worst totally ignorant.
(obsolete) Unknown; undiscovered
Example: 1845, Robert Browning, letter addressed to Elizabeth Barrett Browning, cited in Percy Lubbock, Elizabeth Barrett Browning in Her Letters, London: Smith, Elder & Co., 1906, Chapter 4, p. 106,
[…] as to you, your goodness and understanding will always see to the bottom of involuntary or ignorant faults—always help me to correct them.