You can mark messages for a certain action, such as deletion or
move. After one or more messages are marked, you can then execute
(mu4e-mark-execute-all, x) these actions. This two-step
mark-execute sequence is similar to what e.g. dired does. It is how
mu4e tries to be as quick as possible, while avoiding accidents.
The mark/unmark commands support the region (i.e., “selection”) — so, for example, if you select some messages and press DEL, all messages in the region are marked for deletion.
You can mark all messages that match a certain pattern with %. In addition, you can mark all messages in the current thread (T) or sub-thread (t).
When you do a new search or refresh the headers buffer while you still
have marked messages, you are asked what to do with those marks —
whether to apply them before leaving, or ignore them. This
behavior can be influenced with the variable
mu4e-headers-leave-behavior.
For more information about marking, see Marking.