What it means
Spot forex is OTC, so each dealer quotes its own prices. That is why two brokers' EUR/USD charts can differ by a fraction.
In practice
In an OTC market you trade with a dealer or broker at the price they quote, instead of with an anonymous counterparty on an exchange. Spot forex, most CFDs and many bonds work this way.
The practical effects are that prices and spreads vary between providers, there is no complete record of volume, and your contract is with the firm you trade through.
What to watch for
Your protection depends on the dealer: its regulation, its financial strength and how it executes orders. The volume shown on an OTC chart is that broker's own activity or a count of price changes, not the whole market.