Example sentences
wǎn晚shàng上yì一qǐ起chī吃fàn饭？
Dinner together tonight?
wǎn晚ān安！
Good night!
About this word
晚 is "late" and 上 is a suffix that places a thing inside a stretch of time, so 晚上 is "in the late part of the day", covering evening and much of the night. The same 上 builds 早上 (morning), 上午 and 下午 (before and after noon). It is a plain noun, so a date goes in front of it: 今天晚上, 昨天晚上, 明天晚上. 晚 on its own means "night" in 晚安, "good night".
The story behind the character
晚
晚 is 日 (sun) with 免 on the right for the sound. 免 means to avoid or to spare, but here it's only telling you how the word sounds. Xu Shen defined 晚 as 莫也, and 莫 is the old character for dusk. The sun going down. That's 晚上, the evening.
上
A line with a short mark above it, meaning "on top of." This kind of character isn't drawn, it's indicated. Xu Shen wrote 上, 高也: "up, high." Fun fact: it used to be two lines, and the vertical stroke was added later to stop it being confused with the number 二.