I'll try ...
Basically, BA avios are used like currency to buy a seat.
Each seat costs x amount of avios plus you pay the tax and other fees in real money. This ranges from as low as 35k avios + low fees for something called a reward flight saver (generally European locations) economy return up to 300k avios + I guess around £1200 for a first class return to Sydney.
However, you can only spend the avios if there's availability on your chosen flight for seats that can be purchased with avios currency. BA only release a small amount of seats that can be purchased with avios per flight and availability is often very limited to popular destinations or at peak time. If they only designate 2 seats per class to avios purchasers and someone books them then they rarely offer any more. You can book from 355 days in advance.
You can also use the BA avios to buy hotel nights or car parking (I think) but the relative cost of those is high - most people would see that as a waste of avios unless you can't spend them any other way.
Lately, BA have started to offer money off real flights in return for Avios I.e. £300 discount for say 30k avios.
You can only really see what your options are if you log into your BA Exec Club account and start to play around with the 'how to spend avios' section. Alternately, you could call the BA Amex line and ask them.
Am just about to cancel my Malaysian flights in November so there will definitely be a pair of first class return tickets becoming free shortly!