It means a little more than 180cm(一米八), which can be any thing from 180cm to 189 cm. People use it at the time when they are not sure about the certain number. It can also be used as ‘几百米’ (few hundred meters), which is also when people don't know about the accurate amount. Hope I answered your question!/ -Hi!This is Jiatong or call me Tessy! I am a Chinese teacher on italki and I speak fluent in English and Italian. Take my class if you want to improve your Chinese in a casual way!

就是一米八多，要看提问者的口气 是提问还是描述，描述就是形容这个人一米八多，提问就是问中国人身高到底是一米八几？。。。(๑⌃ٹ⌃)

几 is usually used for undisclosed digit in ones when talking about numbers, from 1-9.