二 is two horizontal strokes — one line for 'one' (一), two lines for 'two'. The simplest counting system in the world: just stack the tally marks. 一, 二, 三 follow the same beautiful logic.
二 is 'two', but spoken Chinese often prefers 两 (liǎng) when counting things: 两个人 (two people), not 二个人. Use 二 for numbers themselves — 二月 (February), room 二〇二. Oddly, 二 alone can also be slang for 'silly' in the north.
Two stacked lines = TWO. One more than 一.
When counting actual things, Chinese swaps 二 for 两 (liǎng): 两个 (two of something), not 二个. But months, dates, and bare numbers stay 二.