第 (dì) is the ordinal-number prefix:
第一 first,
第二 second,
第十 tenth,
第一百 hundredth,
第一千 thousandth. The number itself is built normally — yī qiān ('one thousand'), bā bǎi líng wǔ ('eight hundred and five') — and
第 just goes in front to make it ordinal.
