The Account's token that is found under the Cogwheel --> General --> Account Settings is used when shipping logs to Logz.io. Every log or batch of logs must have this token appended so that we know that the logs belong to your account. The Account's token is referenced in the Log Shipping instructions included within the Log Shipping page within Logz.io.
The Shared Tokens found under Cogwheel --> Shared Tokens are used for sharing visualizations and dashboards publicly. Here you can create a token for each user with whom you want to share a visualization or dashboard. To use the token while working with a visualization or dashboard in Kibana, click on the Share button in the top right menu of Kibana and then you will see the different ways to share that visualization or dashboard. When using the "share publicly" option, you will have to specify which Shared Token you want to use for that share.