(I don't know much about Soros in particular, but I've got an armchair-economist understanding of part of your question.)
'Speculation' when it comes to the financial market is the act of buying something when the cost is low, with the intent of selling it when the value is higher. It's the same thing as what you do when you buy stock in a company, or in precious metals. On a large or publicized scale, though, it can damage the public perception of the commodity in question: 'Why is Bugs Bunny selling all his stock in Acme?' 'Acme must be peaking and ready to start declining. I'd better sell my stock before that happens.'
With currency, it can be done fairly easily (assuming you have bank accounts in different countries) by simply converting money from dollars, to pounds, to yen, and so forth. The fluctuations in the exchange rate work something like the stock prices. If a currency 'value' is falling or about to fall (like the pound did after Brexit), and someone with billions of dollars in that currency shifts it all to another currency, it acts like a large 'sale' of stock. 'Why is Lex Luthor taking all his money out of Latverian banks?' 'Something must be up. We'd better convert our money as well.' (You can also see this on a smaller scale in 'Mary Poppins', when Michael Banks sparks a panic when his father wants him to deposit his tuppence, which he wants to use to feed the birds. The other customers see a bank employee refusing to give a child his money, and fear for the accessibility of their own deposits.)