When we refer to the senses, we are talking about how the author or poet uses imagery to make what they are describing come alive to us by appealing to the five senses: sight, touch, taste, smell and hearing. Obviously it is slightly difficult to use all of these in literature, but good images will try to appeal to more than just a visual description of what is going on. In this excellent poem, which uses an extended metaphor of firewords to describe the love-hate relationship between the speaker and her partner, note how the senses are appealed to:
But whenever I see you, I burst apart
And scatter the sky with my blazing heart.
It spits and sparkles in the stars and balls,
Buds into roses--and flares, and falls.
Note how we have a strong visual description of the speaker "bursting apart" on the sight of her partner. The firework of her "blazing heart" is said to "spit and sparkle," giving us the sound of what happens, combinesd with the "stars and balls" transforming into "roses" as they explode. Through such appeals to the senses Lowell has created a very strong poem full of images that, by appealing to the senses, allow us to imagine and see what she is describing in our mind's eye.