A mailing list is based on e-mail and works as a means of mass mailing all those subscribed to the list. Once subscribed (or registered), which involves (usually) sending a message to a subscription e-mail address with a subject line as used by the list, any messages sent to the submission address as used by the list will be sent to all subscribers.
To post a message you create a message using your e-mail software (sometimes called e-mail client) and send to the address used by the list. In many cases (all except those of us with web-based e-mail) this has to be done from the computer that has your e-mail account details.
You can usually look at archives of messages submitted to the list previously. This is a good idea, since this allows you to check that the list actually has subscribers sending messages that are of interest and use to you.
You may wonder whether it is possible for professional spam collectors to retrieve your email from the mailing list. Well, the archive of the mailing list does not show your email as text. Instead, an image (jpeg) gets created from your email. Spam collectors would have to apply some OCR technique to "read" your email address from the mailing list archive. This is practically (almost) impossible due to the large amount of images.