That's the opening auction. Trades start in staggered lots alphabetically by ASX code.
ASX OpenOpening starts at 10:00 am (Sydney) and lasts for approximately 10 minutes.
Group 1 -10:00:00 am Letters A-B (e.g. ANZ)
Group 2 -10:02:15 am Letters C-F (e.g. CBA)
Group 3 -10:04:30 am Letters G-M (e.g. GNC)
Group 4 -10:06:45 am Letters N-R (e.g. NAB)
Group 5 -10:09:00 am Letters S-Z (e.g. SEK)
There's also a closing auction, but all ASX codes finalise the auction at 10 minutes past the ASX end of normal trading (usually 4:10 PM).
You can add and remove offers and bids at will during the auction.
Both are often used by traders trying to buy for a pip less or sell for a pip more. Involves some luck
Trading Hours (ASX & Global) - Market Index