A platform is nothing more than a trading software that works with specific technology that allows a money manager to trade a large group of separate accounts as if they were one.
Portfolio managers use various platforms to manage money. The particular platform utilized depends on the one offered by the brokerage firm where the accounts are managed.
In forex, the most common managed platform is MetaTrader 4 (MT4) made by MetaQuotes. MT4 is considered by some the "Windows" of FX; in other words, the closest thing to a "standard" in the forex industry. MT4 allows managers to trade using discretion (manual trading) and also automation. The MetaTrader 4 platform has its own programming language known as MQL (MetaQuotes Language). Many investment managers use MQL to code their trading rules and methodologies into either black-box, algorithmic systems or customized indicators for systematic use with discretion. MT4 also allows traders to program advanced order-handling rules into their trading.
One of the reasons MT4 has become such a popular platform is because it is very "visual" in nature. MT4 charting is very popular with both aspiring traders and professional money managers alike for their ease of use and flexibility.
MetaTrader 4 is also convenient for investors because it provides them with real-time access to their accounts. This allows investors to either see the actual positions traded or the real time P&L of the traded positions themselves.