The Current Sessions utility is one of many offered under the Diagnostics panel, found in the web interface. By simply hitting the search button without adding filters, it will display all sessions open through the device. Filters can be customized to display particular results, such as filtering by protocol being used, source/destination IP addresses & ports, which direction the sessions are going (Either inbound or outbound), along with the status and type. Filtering by status and type is a bit more particular, and in general these filters are rarely used but are included as advanced options.
Using this utility can help troubleshoot various issues. For example, if experiencing trouble with a VPN connection, use this utility to see if there are any sessions in the table referencing the destination IP addresses or ports being used, and whether they are going out the correct WAN line using the correct IP address.
Another use for this is to see if there is any odd traffic coming into the network. For example, checking what type of sessions are reaching a public web server. The utility displays what sessions are going to and coming from the device by filtering for its IP address. If traffic other than port 80/443 traffic found and is unexpected, that may indicate in issue for investigating further.
The Current Sessions list is very useful when used in combination with the Traffic Dump utility. These tools provide further insight into what types of traffic is traversing the network.