Refers to the time 2:00, either AM or PM depending on context.
会議は２時から始まります。
The meeting starts at two o'clock.
今、
It's two o'clock now.
二時 means 'two o'clock', a homophone of 二次 that learners often confuse when listening rather than reading.
Standard modern spelling using Arabic numeral.
Kanji numeral spelling; common in formal or vertical writing.
Compound of 二 (に, 'two') and 時 (じ, 'o'clock').