The Signals stack allows you to look at a single Signal in more detail, providing additional information such as organizations and people mentioned in the Signal.
Clicking on a Signal from the list of Signals in Insights (see 1 below) or from the Signals Feed in Explorer, opens the Signal in the stack on the right-hand side (see 2 below). 
In the stack view of the Signal, additional information about the Signal is displayed. Such as meta-data specific to the Signal type and detail that are mentioned in the Signal like topics, organizations, people, or places (see 1 below).
Clicking on the tags or the filter icons will add that value to the filter of your search field, which allows you to easily discover similar topics and Signals (see 2 above).