是 (shì) can function as an interjection expressing affirmation or agreement, similar to 'yes' or 'right' in English. In this usage, it stands alone or begins a response to confirm what someone has said, distinct from its more common role as the verb 'to be.'
"你是李明吧?""是,我就是。"
"nǐ shì lǐ míng ba?" "shì, wǒ jiù shì."
"You're Li Ming, right?" "Yes, that's me."
"明天九点开会。""是,我知道了。"
"míngtiān jiǔ diǎn kāihuì." "shì, wǒ zhīdào le."
"There's a meeting at nine tomorrow." "Yes, understood."
"这份文件需要今天交。""是,马上就办。"
"zhè fèn wénjiàn xūyào jīntiān jiāo." "shì, mǎshàng jiù bàn."
"This document needs to be submitted today." "Yes, I'll take care of it right away."