It means "from some point in the past up till the present time"
For example "I moved to this town in 2001 and I have been living there since then".
2014年11月26日
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'Since then' can be considered an adverbial time phrase. It means 'from a time in the past until now' . For example 'When I first started my job three years I was very unsure of myself. But since then I've become much more confident'.