Well I'd like to see the context of the question, and if he spoke any further to see if it's not being misleadingly cut.

Here's the source, which shows that this is not recent, and in fact came from the 2015 election campaign.

It's a lot to transcribe, so paraphrased they say:

Clark's Question: You are running on a campaign to run deficits. Current Government (Harper), has had 6 deficits. What exactly makes your deficits good, and theirs bad?

Trudeau's Response: Deficits are a way to measure growth and success of a government. Harper's growth rates are awful. Harper promised to not run deficits in 3 campaigns, and then still ran deficits.

It then transitions into the campaign talking points, deficits as investment, low interest rates, economy is flat and needs investment, etc.

What did Justin Trudeau mean when he said deficits are a way of measuring success? What is he talking about? Is he admitting his government is failing or what?

In order: Not 100% sure, but it seemed like it was a statement of how effective deficit spending was, and how much growth comes from it reflecting on the government. He's also not admitting his government is failing, as it's from over a year ago when Trudeau did not have a government.