Forward engineering refers to the process of taking a less-complex abstraction and using it to build a more complex implementation.
Forward engineering refers to taking a high-level model and using it to build a more complex lower-level implementation (as opposed to reverse engineering where you take a complex implementation and try and convert it into a higher level abstraction).
Some examples may clarify it. Forward engineering is taking an EER (Enhanced Entity Relationship Model) or Database Diagram and having software convert the diagram into physical database tables. Forward engineering is taking a UML (Unified Modeling Language) diagram, and converting it into concrete classes.
For reference, an example of reverse engineering is taking compiled machine code, and converting it into human-readable code, such as C++ or Java.