A strike is when you withhold your labour in an effort to extract concessions from the people for whom you provide that labour.

No part of that actually requires an employment relationship. Volunteer strikes are not nearly as common as employee strikes are, but they’re not all that uncommon either. They just require that the volunteers are providing, in the form of their labour, a significant amount of value to the organisation against which they are striking.

You may remember that Reddit moderators did it in response to admins removing API access. On that occasion, it failed in no small part due to a lack of discipline in the strikers themselves.

In Wikipedia’s case, it’s due to the Wikimedia Foundation disbanding the team responsible for dealing with the least of Wikipedia editors’ feature requests, in favour of distributing that work across its regular dev teams. (Editors are volunteers, but developers are paid Wikimedia employees.) The fear is that these employees will inevitably prioritise their own internal work over the feature requests of editors, so features that editors are asking for will not be delivered. The degree of success will largely depend on how many of the highest-volume editors participate, and whether average, low-volume editors (a) join in in solidarity, and if not, (b) are able to pick up the slack.