The original meaning was basically what the meaning is today, unless you have a different sense of the meaning today than I do.

The symbol represents dualism. The simplest way to look at it is dark (陰 yin) and light (陽 yang). The terms as describing forces in opposition are fundamental to a Taoist outlook on life, and a core part of Chinese philosophy.

A well known verse (chapter 40) of the Daodejing 《道德經》 says that "reversal is the movement of Tao" (反者道之動). The idea behind this is that there will always be a cyclical back-and-forth. The turning of the yin-yang symbol represents this. Imagine it spinning, going from light to dark and back again. Throughout the Daoist texts you find references to this sense of dualism.

Two centuries later there was also a scholar in the Warring States period named Zōu Yǎn 鄒衍 (died 240BCE) who headed the Ying-Yang School (陰陽家). This was a philosophical school/creed which attempted to explain the world in terms of these opposing forces, yin and yang. The idea of opposition and alternation between yin and yang were fundamental to their worldview.

The notion of yin and yang predate the Laozi, but it hasn't always been with the symbol you have in mind. The trigrams/bagua that you see on the Korean flag are based on the idea as well. Where the line is broken that's yin, and where solid it's yang. The combinations of broken and solid represent the elements, so ☲ which is yang, yin, yang, represents the element of fire or, in other contexts, the sun. The Korean flag, showing four positioned around a version of the yinyang symbol (taegeuk, 태극, 太極), is heavy in Daoist symbolism, despite the strong Confucian orientation of the peninsula for the past many centuries. The four trigrams are heaven ☰, earth ☷, the moon ☵ and the sun ☲. These all go back to the I Ching (yi jing), an older text on/for divination which later became significant to the Daoists.

tl;dr: It originally meant what you think it means now.

Further Reading: I recommend A Short History of Chinese Philosophy by Feng Youlan (馮友蘭) if you want something that will cover a long time span but also cover a wide range of topics, including the early Daoists. I've not read the English translation but at least the Chinese version is an easy read and gives a good overview of Philosophy in China. I can't imagine the English translation somehow messes that up.