What is a Spike in Scrum?

Spikes are a type of exploration Enabler Story in Scaled Agile Framework (SAFe).
Defined initially in Extreme Programming (XP), they are used to gain the knowledge necessary to reduce the risk of a technical approach, better understand a requirement, or increase the reliability of a story estimate.
A spike has a maximum time-box size as the sprint it is contained in it. Spikes are investigation activity (such as research, design, investigation, exploration, prototyping) to gain the knowledge to solve a problem.
They produce information, rather than working code.