The "Drafts" feature of phpBB3 (this forum's software) is a very helpful feature. It is meant to save a draft of your topic OR reply before you submit it. You can save your post as a draft in case you have to leave quickly, or just want to finish it later!
To save a post as a draft, click the "Save" button instead of "Submit". It will ask you for confirmation, and then it will save your draft into your UCP. Please note that this draft is a completely private thing and no one can see it until you hit Submit within your UCP.
So - you've saved your draft. Now what? Well, when you come back to MLKF to complete your post, click "User Control Panel" on the left side of the nav bar. Then click "Manage drafts" within the "Overall" category. From there, you have several options. Under the "Options" column, for each draft, you will see "Load draft" and "View/Edit". "Load draft" will load the draft into the topic or forum in which you were working on it beforehand. "View/Edit" is for quick edits. You can use it when you know you won't be submitting your draft as a post. When you're ready to submit your draft into a post, click "Load draft" and then "Submit" as you would regularly do.
Please note* Drafts do not currently save attachments or other elements besides the post title and the post text.