A Blast is a message you send to a group of people at once rather than one at a time. To get one, tap the coin symbol in the top right corner of the Feed page to open the Rewards page, then choose how many people you want to reach:
20 users — 75 coins
50 users — 175 coins
100 users — 315 coins
You can also get a Blast for free as a reward for keeping a 7 day streak, which renews every week.