Events are something that happened to or within an object (website or application). In general, you would treat them within the object's own class code. for instance considering the scenario of a website, page view, add to cart, etc are events.
Signals are emitted by an object. The object is basically notifying other objects that something happened. Other objects might do something as a result or not. in our case every time an event like page view takes place on the website a signal is fired to the marketing platform like Facebook or Snapchat. Signal is any event coming from a source (website here) and going into any destination.
taghash plans depend on signal volume of each platform. If one event is sent to two platforms, we count two signals.
In case of data being collected from online sources like CRM, Cloud or FTP, every line if information being uploaded is considered as one signal.