Spike (concept originating from Extreme Programming) may be considered a type of a Product Backlog Item that is usually focused on research, exploration and investivation activity.
The purpose of a spike may be to explore the technical feasibility or create a quick proof of concept to gain better understanding about a particular requirement.
Since a Spike does not directly deliver stakeholder value, and due to unknown nature of the work involved, a spike should be subjected to a time-box.
The knowledge gained during this time-box (effort spent) should be presented at the Sprint Review and based on it the Product Owner may decide whether to puruse this feature / technology further or not. If the Product Owner decides to pursue it further, they may write a fresh Product Backlog Item and add it to the Product Backlog.