As stated in other answers, in English, the tilde is almost always placed before a number or a time or date to denote a short hand that the value is an approximate, not an exact, value. In the following sentence:
John Doe was born ~1850 and died August 1st, 1901. He was large for his age, which helped when he lied about his age when he signed up to join the Union Army in 1864. His mother was still shocked that the union would let a 14 year old boy go to war.
The read is that the author does not know when John Doe was born to a specific date, but does have documentation of the exact date of his death. John Smith could have been born prior to 1850 or some time after, but he was of an age that he would have been a 10-15 year old boy during the course of the U.S. Civil War (1860-1865).
Generally, its used in non-fiction and in notation rather than directly in text. In legal documentation, Lawyers prefer to use the phrase "on or about" or "about" for reliable guesses. This allows for testimony where the exact value is not known but the general value is consistent. A person can say ~10 people when in fact there were 8 because their memory is unreliable and not everyone will count out the exact number of people involved in an event.