To be 100 percent transparent, I struggled through researching this topic. However, what helped me understand it was this:

• Talking it through with a software engineer that I respect (Chase Baker)
• Breaking down the definition into very small pieces

With that in mind, let's walk through a dead simple definition of the concept. The LSP states this:

A program should have the ability to replace any instance of a parent class with an instance of one of its child classes without negative side effects.