Slim fit is a closer clothing cut that works best for slim or slender bodies. Slim fit garments taper towards the narrower parts of your body. The legs narrow towards the ankle, sleeves narrow towards the wrists, and side seams narrow towards the natural waist.
However, slim fit does not mean skin tight. Slim fit jeans, for example, will not fit as snuggly as skinny jeans do. There should still be enough room in the garment for you to move around comfortably.