In Yo-kai Watch Blasters, you can play as Yo-kai (only four medal slots aside from the main game's six slots) from YO-KAI WATCH 1 and YO-KAI WATCH 2, including the Yo-kai Usapyon (Japanese: ＵＳＡピョン USApyon) from YO-KAI WATCH 3. You play missions (you need to progress in the game to unlock them all), which come in three types: Story, Patrol, and Boss Rush (Big Boss in the English-language versions). In all these missions, you can collect Oni Orbs, which are very important in the game. You can use Oni Orbs to level up Yo-kai, buy items, make and upgrade equipment, use the Oni Crank-a-kai, etc., and they can be found in most missions you take part in.
In Story Missions, you can progress through the story, to become the best Blaster you can be. There are two types of Story Missions: Main Missions and Sub Missions. As the name suggests, Main Misisons are essential to completing the story. In Sub Missions, however, there are some of these which are essential to progress in the main story, and some are completely optional and can be done separately. In some of these misisons, you can get certain Yo-kai.
In Patrol Missions, you can explore different towns of your choice, although in Springdale, you can choose the location you want to patrol in. Many rare Yo-kai that you are able to befriend spawn here. It is recommended that you befriend Yo-kai in Patrol Missions. You can do Mini Missions, which are tiny missions you can do to receive Oni Orbs.
In Big Boss Missions, you can fight bosses you fought in the story already. There are difficulty levels in these Yo-kai as well. You can fight the bosses in Normal difficulty, Super, and Ultra which get harder every time. In Ultra you can receive rare items OR maybe even befriend the boss.
You can only complete missions if you complete the mission's respective objective. These may include collecting items, battling Yo-kai, etc. If all the Yo-kai in your team faint, or the timer runs out, you fail the mission. You control the leader Yo-kai (whoever you place at the red section of the Move-out Members menu), and you can switch between the other Yo-kai in your team. Every Yo-kai has a role, and each role is depended on what your Yo-kai will do throughout the mission: Fighter, Tank, Healer, and Ranger. It is certainly recommended to balance your team with these four roles.
Source: Wikipedia, "[en.wikipedia.org/wiki/Yo-kai_Watch_Blasters Yo-kai Watch Blasters]", available under the CC-BY-SA License.