て以来 means 'since (that event)' and is used to describe something that has been true continuously or repeatedly from that point in time up to the present. It often carries a nuance that the event was a turning point. It's commonly used with time expressions like ずっと (all along) or 初めて (for the first time).

This pattern is more formal than から, but still used in both speech and writing. However, it is not used for one-time events in the past; it emphasizes the continuation or ongoing relevance. For example, you wouldn't say 去年日本に来て以来、富士山に登りました (since coming to Japan last year, I climbed Mt. Fuji) because that's a single past event. Instead, use it with present or present-perfect-like states.