If topics have been set up by your mailing list administrator, you can choose to subscribe to only part of a list by selecting the topics you want to receive.
If you wish to get all messages sent to the list, make sure you are not subscribed to any topics.
If any topics are defined, you can select those you wish. If you do not select any topics of interest, you will receive all posts sent to the list.
You probably also want to look at Section 9.3 which discusses changing your settings for messages where no topic is set.