直到 (zhí dào) means until or up to a point in time. 直 means straight or directly and 到 means to arrive at. Together they indicate continuous duration up to a moment (直到深夜, until late at night; 直到现在, up until now). It often pairs with 才 (cái) to express only then: 直到那时，我才明白 (not until then did I understand). This structure emphasises the delay or lateness of a realisation.