A morganatic marriage is basically a marriage between a man (usually) of higher rank who marries a woman of lower rank and does not pass any of his titles and privileges to his wife and any resulting children. The purpose being, to allow marriage for love when it otherwise wouldn’t be allowed while still preventing undesirable children from joining lines of succession.
Morganatic marriages were most popular in Germanic countries and Russia. Ghengis Khan also practiced polyamoric morgantic marriage, where only the children from his official wife were allowed to inherit while the children from his morganatic wives were not, though they were still legitimate. Morganatic marriage was never really practiced in the UK because there was no prohibition against marrying commoners in the first place. Edward VIII proposed a morganatic marriage with Wallis Simpson so as to marry her and remain king, however parliament did not approve and we all know how that ended.
Famous Morganatic Marriages:
Archduke Franz Ferdinand and his wife Sophie. Though she was an aristocrat, Sophie was not a member of a ruling royal family, she was not eligible as a royal wife. They ended up having 3 children and were assassinated in Sarajevo, an event that kicked off World War I.
Victor Emmanuelle II of Italy and his wife (and former favorite mistress) Rosa. The first king of united Italy had a proper royal marriage to Adelaide of Austria which gave him 8 children. After she died, he married his favorite mistress in a morganatic marriage.
Marie Louise, Duchess of Parma. After the death of Napoleon Bonaparte, his wife Marie Louise married morganatically TWICE, first to Count Adam Albert von Neipperg and then to Charles René de Bombelles (her chamberlain!)