1. To enter or slot into some place, area, or thing, especially when space is limited. A: "Would you mind giving me a ride home?" B: "Sure, no problem. Go ahead and slide in next to Jeff in the back seat." We'll need to find a spot in the living room where this bookshelf can slide in.
2. To insert or place someone something into a particular spot or location. In this usage, a noun or pronoun can be used between "slide" and "in." The classroom is a little bit crowded, but I'm sure we'll be able to slide you in somewhere. I'm thinking that we'll slide in a dresser between the bed and our wall.
3. To enter into some situation in an effortless or inconspicuous manner. We're hoping the new project manager will be able to slide in fairly seamlessly so that there aren't any further disruptions to the project. You think you can slide in and replace my dad just because you've been out on a few dates with my mom?
See also: slide