Start from a message you have written before, fill in the parts that change per send, and see exactly what each recipient will receive before it goes.
A template is a message you have written once and can start from again. It carries the words, the merge tokens, whether the opt-out footer goes on, and whether the message is marketing or operational, so picking one sets up the whole composer rather than just pasting some text.
There are two kinds, and the composer marks which is which:
Choose a template from the dropdown above the message box. It fills in the message, sets the message type and the opt-out footer, and names the campaign if you have not named it yet. From that moment it is your message: editing it changes this campaign and nothing else.
A supplied template uses tokens that most organisations will not have declared yet, such as {{session}} or {{venue}}. That is deliberate. Either declare a contact field with that name and fill it in per person, or type a value for the whole send in the mail merge panel underneath.
Some things vary per message rather than per person: the date of this closure, the deadline for this reminder, the venue for this event. Type those into the mail merge panel and they apply to everybody on the send.
Where a person has their own value, theirs wins. A value you type here is used for the people who have nothing of their own, which is why the box tells you how many that is:
Message: Your tutor is {{tutor}}.
Typed value: tutor -> your course leader
Somebody with a tutor: Your tutor is Dr Patel.
Somebody with no tutor: Your tutor is your course leader.The panel counts, across everybody the message will reach, how many hold no value for each field it uses. This is the number worth acting on before a send: a hundred people with no tutor is a hundred sentences with a gap in the middle.
You have three ways to deal with it: fill the values in on the contacts, type a value here for the whole send, or tick the box to skip those recipients entirely. Skipping removes them when the send starts, so the audience and the cost you were quoted are the ones that actually go out.
Preview for a real recipient renders the message against actual people from the lists you have chosen, with the prefix, signature and opt-out footer applied, and the segment count for that person. It deliberately leads with whoever is missing a value and whoever holds the longest ones, because those are the two versions worth looking at.
Save as template stores the message under the campaign name, along with any values you typed. Those become the starting values the next time somebody picks it, and they can change them for their own send without affecting the template.