In psychological terms, constructs are "typical" representations of thought-forms that have many possible expressions. For example, the image brought to mind when one thinks the word "dog" is not wholly representative of every dog. It is a generic representation of what you believe to be the most common traits a dog has, along with some added details flavored by your memories.
In psi, contructs are the application of this representation in an energetic thought-form. This can form intents easier by molding energy into an already-established pattern. Fire is easily accomplished energetically, as is ice. Constructs are not limited to visual images, however. Feelings can create empathic constructs of love, anger, lust, etc. These can be helpful in calming down stressful situations (or in having a successful date, just between you and I). Your imagination is truly the limit to what you can create in energy with constructs. The articles that I write following this one will illustrate some of these examples. Best of the energies to you on your path.