It means to discipline them so that you control them rather than them controlling you, your not denying yourself so much as you are denying the senses, denying the body to free yourself from them so that you can truly be yourself as God made you to be.
Mortification of the senses, of the imagination and the passions
1 - Close your eyes always and above all to every dangerous sight, and even - have the courage to do it - to every frivolous and useless sight. See without looking; do not gaze at anybody to judge of their beauty or ugliness.
2 - Keep your ears closed to flattering remarks, to praise, to persuasion, to bad advice, to slander, to uncharitable mocking, to indiscretions, to ill*disposed criticism, to suspicions voiced, to every word capable of causing the very smallest coolness between two souls.
3 - If the sense of smell has something to suffer due to your neighbor's infirmity or illness, far be it from you ever to complain of it; draw from it a holy joy.
4 - In what concerns the quality of food, have great respect for Our Lord's counsel: "Eat such things as are set before you." "Eat what is good without delighting in it, what is bad without expressing aversion to it, and show yourself equally indifferent to the one as to the other. There," says St. Francis de Sales, "is a real mortification."
5 - Offer your meals to God; at table impose on yourself a tiny penance: for example, refuse a sprinkling of salt a glass of wine, a sweet, etc.; your companions will not notice it, but God will keep account of it.
6 - If what you are given appeals to you very much, think of the gall and the vinegar given to Our Lord on the cross: that cannot keep you from tasting, but will serve as a counterbalance to the pleasure.
7 - You must avoid all sensual contact, every caress in which you set some passion, by which you look for passion, from which you take a joy which is principally of the senses.
8 - Refrain from going to warm yourself, unless this is necessary to save you from being unwell.
9 - Bear with everything which naturally grieves the flesh, especially the cold of winter, the heat of summer, a hard bed and every inconvenience of that kind. Whatever the weather, put on a good face; smile at all temperatures. Say with the prophet: "Cold, heat, rain, bless ye the Lord." It will be a happy day for us when we are able to say with a good heart these words which were familiar to St. Francis de Sales: "I am never better than when I am not well."
10 - Mortify your imagination when it beguiles you with the lure of a brilliant position, when it saddens you with the prospect of a dreary future, when it irritates you with the memory of a word or deed which offended you.
11 - If you feel within you the need to day dream, mortify it without mercy.
12 - Mortify yourself with the greatest care in the matter of impatience, of irritation or of anger.
13 - Examine your desires thoroughly; submit them to the control of reason and of faith: do you never desire a long life rather than a holy life, wish for pleasure and well-being without trouble or sadness, victory without battle, success without setbacks, praise without criticism, a comfortable, peaceful life without a cross of any sort, that is to say a life quite opposite to that of Our Divine Lord?
14 - Take care not to acquire certain habits which, without being positively bad, can become injurious, such as habits of frivolous reading, of playing at games of chance, etc...
15 - Seek to discover your predominant failing and, as soon as you have recognized it, pursue it all the way to its last retreat. To that purpose, submit with good will to whatever could be monotonous or boring in the practice of the examination of conscience.
16 - You are not forbidden to have a heart and to show it, but be on your guard against the danger of exceeding due measure. Resist attachments which are too natural, particular friendships and all softness of heart.