With regard to many kinds of crimes or torts, it is necessary for the accused to have had a certain mental state when the act was committed. If the person lacked that mental state, then there is no crime or tort. For other civil or criminal violations, no mental state or intent is necessary. The person committing the act or causing the problem is liable in all circumstances. This latter type of violation is one in which strict liability exists. In short, then, strict liability is when a wrongdoer is held to be at fault regardless of whether they acted carelessly or negligently.
An example of this in criminal law is statutory rape. In cases of statutory rape, the offending party cannot escape liability by saying they did not know the victim was under age. There is no need to have meant to commit the crime. The simple fact of having had sex with an underage person makes one liable for this crime.
In civil law, the classic case of strict liability has to do with livestock that a person owns. If you have cattle and they break through your fence and cause damage, you are liable for the damage. It does not matter if you were careless or negligent in building your fence. Your cattle caused the damage and you are automatically liable.
In both of these cases, we see examples of strict liability because no particular state of mind was required and the people were held liable regardless of their mental states.