Right, as "Lee" said it means "just now", in this case.
막 can have a couple of meanings (as far as I know). One is the one you mentioned, and the other is....."carelessly " or in Korean 함부로. And this one is used QUITE frequently, especially in casual conversation.
An example is
막 말한다 ...
I would say the word 막 and 말하다 are used together quite frequently.
And the only way you would translate this, you would probably have to revert to slang. In Korean it's just an adverb, but the level of emotion that it gives to the expression, you would most likely only find in English slang.
So, a great English slang example would be "I was just talking out of my ass". Obviously (be warned English students) that this is slang, and if you were to say this expression in English you would have to be with the right people to do so comfortably. In Korean however, the word 막, while it is used often informally, would not quite reach the level of slang that the above expression would in English. This is what makes it such an indispensable word in Korean. It is very useful.
I heard a Korean friend of mine talking one day and the sentence he used was "내가 막 이래"
And if you look at that sentence it gives you a good insight into how Koreans talk sometimes. In that sentence, where is the verb? There isn't one. Korean at times, can be a very vague language, that uses emotion to express meaning, over precise meaning. So there is no verb specified here, and it can be interpreted in many ways. It could mean "This is how I talk" "This is how I am" "This is what I do"
Now, regardless of what this person said or did, the point of this short sentence is that the person what doing it carelessly. This shows how important the word 막 is, in the sentence. There are a TON of words like this in Korean, which subjectively add meaning to the sentence. But the one you mentioned I would say is in the top 10. Words like this, make Korean a fun language. Whereas in Enlgish you constantly find yourself trying to carefully articulate the "who" "what" and "when" in the sentence, in Korean these things can tend to take a back seat to more emotional language, with the understanding that both parties already know who did what where.
Another example is
총알이 막 날라왔다
"The bullets were flying from all over the place"
Now, the point of the sentence here is the emotion that the word 막 gives to the sentence. The feeling it gives it chaos. As if bullets are just randomly coming from everywhere.
The best way to study these in my opinion, is to carefully watch how they come up in context, and just keep it in mind the next time you hear them come up. Then you will form a larger narrative in your head for the real nuance of the word. They can be tough~~ ^_^;;