As a non-American, I don't get it. What's a primary and what's a caucasus, and why is this important?
Primary = everybody goes to a ballot box and casts a secret ballot. Or, in the case of Washington, fills out and returns their mail in ballot. The winner is determined by counting the ballots, just like pretty much everywhere else in the world.
Caucus = people get in a room together at a very specific time and argue with each other over which candidate that group of people will support. The winner is determined by counting the number of those groups of people statewide that support one candidate versus another. Since the vote isn't secret, the winner of a particular group of people is often influenced by intimidation tactics. If you can't make it at the designated time, you don't get a vote.
Washington makes things a little harder to get - for some reason that I have no clue about, they do both a caucus and a primary. The caucus is what's used to determine who 'wins' the state and gets the most delegates to select the nominee in July at the party convention. The primary is purely a beauty contest and has no effect on delegate allocation.
This is the second state where there was both a caucus and a non-binding primary, where Bernie won the caucus and Hillary won the primary - the latter of which had as many as 10x the number of participants as the former. These results demonstrate that all caucuses do is give a disproportionately large voice to a vocal minority of die hard supporters who have time to show up at the specific designated location. They're very disenfranchising, partially due to the scheduling issues, and partially due to the lack of a secret ballot - in the case of the latter, everybody knows who you support, and the other side will try hard to get you to switch your support. In that case, peer pressure and intimidation can go a long way.
