You've probably seen someone chanting "kuwabara kuwabara" after a flash of lightning — or done it yourself.
Literal
Lightning [subject-が] flashed that-after, "kuwabara kuwabara" [quotative-と] chanted, or chant person [object-を] saw thing [subject-が] exists [conjecture-だろう].
Explaining a cultural superstition. くわばらくわばら is a traditional chant used to ward off lightning — similar to English 'knock on wood.' The origin is contested, but one story traces it to a field in 桑原 (Kuwabara) said to be struck by a curse-warding deity. ~たことがある ('have the experience of X-ing') + だろう ('probably') = 'have probably seen/done.' もしくは is a formal 'or.' 【】 brackets are used to highlight the set phrase.