A systematic trader writes down what counts as a trading-signal, how big the position is, and when it closes, then follows that specification without editing it mid-trade. The rules can be executed by a computer or by a human with a checklist; what makes it systematic is that a second person given the same rules and the same data would take the same trades.
The advantage is measurability. Because the rules are fixed you can run them over history, count the trades, and estimate an edge. The cost is that the rules cannot see anything you did not anticipate, so a systematic book will keep trading straight into a market it was never designed for until a regime-filter or a human turns it off.
Example: a rule saying buy when the 20-day moving-average crosses above the 100-day, risk 0.5% of equity, exit on the reverse cross, is fully specified. A rule saying buy when the trend looks strong is not, and cannot be tested.
Related: discretionary-trading, trading-system, rule-set, trading-signal