Simply put, 'Optus 4G Plus' is branding for the Optus 4G network. This network covers all major cities in Australia and Optus is building it to cover more areas around regional parts of the country.
Optus add the 'Plus' to its network name when it announced it would add a second band of spectrum to the 4G network. Originally it only used the 1800MHz frequency, but now uses the 900MHz band as well.
This is significant for two reasons. 2 different bands helps Optus spread out its 4G customers and helps with congestion on the busy network. Secondly, the 900MHz band is capable of covering greater distances, so this helps more customers use 4G even though they might be further away from a mobile tower.
You can check Optus coverage in your area by following this link to the Optus website.
To get on the Optus 4G Plus network you need a compatible phone. The majority of phones sold by Optus right now are compatible, but if you'd like to use an old phone you can ask an Optus store clerk if it will work.