A caption can be thought of as a title or label of a table. It provides a brief explanation of the table's purpose. This is helpful for anyone but particularly screen reader users. The caption is the first thing that is read by a screen reader when a table is given focus, allowing the user to immediately understand the purpose of the table.
Whilst a caption is not always necessary, a table should at least have a summary. The summary should be used to expand upon the caption but should not duplicate it.
//An example of a table summary attribute and a caption element
<table summary = "An overview of ticket prices at the Theatre Royal. The different types of tickets are listed in the first column, and the days of the week are listed in the first row. Find the day of the week you wish to visit the Theatre Royal and then decide on the type of ticket you require">
<caption>
Ticket prices at the Theatre Royal
</caption>