Hmm, it depends in what you mean by "summing prices in a table", ie under which circumstances.
You might find this helpful:
FileMaker Pro 15 Help (Summary fields)
Note that were it reads "multiple records", that could of course be all records in a table, or a found set you get as the result of a Find (all invoices in the 1. quarter) or by following a relationship (all line item records related to an invoice record - or, for that matter, a shopping cart).
There is also the Sum() function that allows you to total a field from all records related to a parent.
But a summary field is more versatile: in
Cart --< LineItems >-- Product
you'd need a Count() calculation field of line item prices in both parents - and still couldn't total those prices in a list layout of LuneItemd.
But if you create a Total of: extendedPrice (price x qty) in LineItems, you can not only total those records, but also display that field in both parent tables:
In Cart, you get a total of the extended prices for all products in the current cart; in Product, you'll get a total of that product's extended prices in all carts (which may not be totally useful ...)