Intensity, when applied to a strength training program, usually applies to the load or weight. Load is usually quantified as a percentage of repetition max.
Intensity may also apply to the repetition scheme or perceived effort.
Intensity is often misunderstood when referring to strength training. Rather than think of "HIGH" intensity in terms of LOAD, many people think of "HIIT" or interval type training which would be based more on perceived effort rather than load.