Sic
[sic] is often mostly pointing out that this is not an error in transcription, it is intentionally reproduced exactly.
It's mostly used around literal quotation, acknowledging to the reader that this might be unusual, archaic, or bad grammar/spelling but we're leaving it as-is.
It is sometimes also used for things that are surprising for other reasons - sufficiently weird reasoning, or things that seem to come out of nowhere.
It seems discouraged to use this for dialects and other less-usual-but-entirely-established use.
The square brackets you often seem to see this in seem to be a convention of editors to mark that they altered something inside the text (also e.g. used to make mild edits to make the grammar work in the context, insert [...] to note omission, etc.)
Some style guides also insist on italicising it.
sic itself just means 'thus' and is used in various phrases.
In this case it is a shorthand for sic erat scriptum (something like 'thus was it written'), but almost no one would know that.