What does union jack mean and where it cames from?

The Union Jack is the national flag of the United Kingdom, symbolizing the union of England, Scotland, and Northern Ireland. It combines elements from the flags of these nations: the red cross of St George for England, the white saltire of St Andrew for Scotland, and the red saltire of St Patrick for Ireland. The flag was first officially used in 1801, when the Kingdom of Great Britain and the Kingdom of Ireland united. Its design represents the historical and political union of these countries under one monarchy.