所 + V (的): that which…
助词 / particles
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
(S) + 所 + V + 的 + (N)
所 before a transitive verb creates a phrase referring to the object of the action: 我所知道的 = "what I know". It doesn't change the meaning (我知道的 means the same) but makes it more formal and precise.
Very common expressions: 所有 (all), 所谓 (so-called), 所在 (where something lies), 众所周知 (as everyone knows), 有所 + V (to some extent).
Examples
据我所知，这座寺庙建于唐朝。
Jù wǒ suǒ zhī, zhè zuò sìmiào jiàn yú Tángcháo.As far as I know, this temple was built in the Tang dynasty.
历史书上所写的，并不都是事实。
Lìshǐ shū shang suǒ xiě de, bìng bù dōu shì shìshí.Not everything written in history books is fact.
众所周知，丝绸之路连接了东方和西方。
Zhòng suǒ zhōu zhī, Sīchóu zhī Lù liánjiē le dōngfāng hé xīfāng.As everyone knows, the Silk Road linked East and West.
Common mistakes
✗ 所 + intransitive verb: ✗ 我所哭的 doesn't work.
✗ Dropping 的 before a noun: ✗ 我所看书 → ✓ 我所看的书。