Say you want to count all the trees in a large forest. You could hire a team of ten people, equip them with hiking boots and click counters and send them on their way. The problem is, it would probably take decades.
Alternatively, you take some high res satellite photos of the forest, create a simple online interface and ask the public to help. The job could be done in a few months. You’ve just crowdsourced your problem. You’ve divided a large task amongst a large audience.
Wikipedia is a crowdsourced encyclopaedia. Kickstarter is a crowdsourced funding body. Zooniverse is a crowdsourced science project.