This option allows setting a limit for opening orders of new symbols. This is particularly useful when you have the expert advisor working in many symbol charts but do not want to have orders opening for every single one concurrently.
Say you have 6 symbols with the expert advisor attached to their charts but only want to have 3 symbols trading simultaneously. If you set
Max Concurrent Symbols to 3, orders may open for 3 different symbols from those you have (no matter which) but when that limit is reached, no orders for new symbols can be opened unless one of the previous symbols gets all orders closed, in which case, orders for a new symbol can now be opened again.