Arb is short for "arbitrage" In the real world, arbitrage is taking advantage of a price difference in two different markets. Applying this to HSX, it means trading where HSX prices do not reflect earned box office in the real world.
Arb usually means that a moviestock can only delist higher than its current price, on the basis of earned box office. In other words, you can make money by investing at the current price, without any risk at all. Arb also applies to some derivatives, like blockbuster warrants.
Only differences between HSX prices and earned box office count as arb, so differences in price between related HSX securities - such as a moviestock and an opening warrant for the same movie - do not provide arbitrage opportunities.
Similarly, there's no such thing as "almost arb" or "will be arb after the weekend". This is because between now and the time when the earned box office matches the current price, the moviestock can (and probably will) change.
Traders will sometimes post that a moviestock is arb on the forums. If you want to invest on this advice, you should first check the moviestock price against the earned box office to verify that the moviestock is still arb. Otherwise you'll be left holding an over-valued moviestock.
If you choose to post that a moviestock is arb, you should ensure that the moviestock is not arb by a notional amount, like a couple of pennies. It may be technically correct, but as information it's not very valuable to other traders, and it will give the impression you are trying to manipulate the moviestock's price.
Less frequently you will see the reverse, where a moviestock can only delist lower than its current price. This is called "bra". A moviestock (or warrant) can really only be bra on its last day of active trading, after actual box office numbers come in. This is because the movie could always surprise and earn more box office than expected.
Traders also apply arb and bra to starbonds, but for adjusts not delists.
A starbond is arb when its current price is lower than it can possibly adjust - calculated either by the earned box office to date, or assuming earned box office of a dollar.
A starbond is bra when its current price is higher than it can possibly adjust. And as with moviestocks, this happens much less frequently, and really only applies after actual box office numbers for the weekend before the starbond's adjust are reported.