adverb
(not) so; (not) particularly; (not) very; (not) that much
Used with a negative verb to express that something is not to a great degree. Often translates as 'not so', 'not particularly', or 'not that much'.
この映画はさほど面白くなかった。
This movie wasn't particularly interesting.
彼の説明
His explanation isn't that difficult.
あまり is also used with negatives to mean 'not very', but is more common and can be used in a wider range of contexts. さほど is slightly more formal or emphatic.
それほど means 'to that extent' and is often used in negative sentences similarly to さほど, but それほど can also be used in affirmative sentences.
Compound of 然 (sa, 'so') + 程 (hodo, 'degree'). The kanji 然 is rare, and the word is now typically written in kana.