P/L Manager automatically manages manual positions, or orders which have been orphaned by an EA refusing to run. You define either (or both of) a stop-loss and profit-target for all the positions, and they are automatically closed out when the target is hit.
The MT4 P/L Manager can monitor a basket of manual orders, or take over the management of trades which have been orphaned because the EA which created them is no longer available.
The orders are closed once their combined profitability hits a defined stop-loss or profit-target. The monitoring can be applied to a single symbol, or to all orders, and can be restricted to the trades from a particular EA (based on the magic number it uses).
P/L Manager can also be used simply as a reporting tool: a way of displaying the current profit on all the orders for a single symbol or EA.