The consultation is what triggering article 4 means. Article 5 is collective defense.
Article 4
The Parties will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the Parties is threatened.
Talking before this; hey what's our plan if Russia invades? Ok, what about if Russia does a "small" invasion? Etc. "Informal" in that they're not producing anything that requires a vote or action from member parties - it's meeting of established joint committees, distribution of memoranda, back and forth on policy.
Talking now: which of the plans we have discussed before are we going to enact? Formal meeting, in that a member state has declared it's intent to bring a topic to the table and that the body is obligated to discuss and determine a course of action - which is allowed to be "no action".
Basically, before now everyone figured out what to do on paper, now the Treaty Organization has to make a decision about next steps.
And as per the Tweet itself, it's a consultation