The all ords is an index made up of the largest 500 companies on the ASX. (Honestly, I'm not sure how we made it in, given how much our market cap has decline, but anyway...) An index is just a way of gauging how a certain group of stocks is performing.
Now that we are part of the all ords, index funds (funds people can invest in which are based on the index) will sell the stocks that fell out of the all ords and buy the ones that entered it.
Also, people anticipate these changes and buy sell accordingly, thus increasing the effect.
AC8 Price at posting:
40.5¢ Sentiment: Buy Disclosure: Held