What are collective management organisations?

They are licensing bodies which grant rights on behalf of multiple right holders and collect royalties on their behalf from use of their works.

Affiliating yourself with a society and registering your songs with it allows you to receive royalties from the use of your music in public.

The main music rights organisations are:

(1) Mechanical collection societies - they represent and license mechanical rights on behalf of their members (writers, composers and publishers) and collect royalties for the reproduction of their members' works (e.g. when their songs are manufactured and sold on CDs, LPs or used online whether as downloads or streams);
(2) Performing rights organisations - they represent and license performing rights on behalf of their members (writers, composers and publishers) for the public performance and broadcast of their members' works (e.g. when their songs are played on internet or terrestrial radio, online, performed at live venues or clubs, played in business premises as background music or broadcast on TV);
(3) Neighbouring rights organisations - similar to the performing rights organisations on the publishing side - they represent and license rights in the recorded music (sound recordings) played in public or broadcast on TV or radio on behalf of their members (performers and record companies). Affiliating yourself with a society and registering your songs with it allows you to receive the applicable publishing royalties or recorded music royalties (as applicable) from the use of your music in public.