Many political parties have members elected to the Australian Parliament. Two of these parties currently have lots of members elected and they are called the major parties. But there are lots of smaller teams with some members elected. They are called minor parties.
Minor parties are able to form government if they have the support of the majority of members in the House of Representatives. They can do this by forming a partnership (coalition) with one or more other parties. Currently, The Nationals (a minor party) are in government in coalition with the Liberal Party (a major party).
The opposition is the largest party or coalition of parties that does not have the support of the majority of members in the House of Representatives. An opposition could be made up of a minor party in coalition with a major party or another minor party.