What's a spike?
A spike is work that typically doesn't result in shippable functionality, such as a feasibility study, technical experiment, proof of concept, or similar. The duration of a spike is typically agreed between the product owner and the team, and the estimate of the spike is set accordingly.
Unlike other types of backlog items, spikes will typically not deliver tangible, shippable, valuable functionality. For example, the goal of a spike might be to reach a decision on a course of action. The spike is over when the time is up, not necessarily when the goal has been reached.