(if you're familiar with Dota over the last few years, skip the first two sections)
So Valve is hosting a Major in Boston, MA, USA, with a prize pool of ~$3 million (just in case you hadn't heard).
As with all Valve events post-TI4, some teams are directly invited to the main event ("Direct Invites") and some are invited to the "Regional Qualifiers" (RQ). Every other team in the world (or random collective of 5 casters) can sign up for the "Open Qualifiers".
Open Qualifiers