A shard is an instance of Yoshino that handles an x amount of servers. To split the load that Yoshino gives the Discord API, all the servers that Yoshino handles are split on multiple instances. To find out what shard your server is on, use the !info command. Usually crashes are shard-wide, meaning if an error occurs and Yoshino crashes, in most cases the entire shard will lose access to Yoshino. A shard can only be restarted by a Bot Developer.