You have to be careful with songs because the lyrics are often poetic or figurative. They aren't literal so it might be hard for anybody to understand what they mean.
When an adverb like 'away' is added to a verb like 'slide' it sometimes gives the direction of the motion of the verb. 'Slide away' means to slide farther in any direction from something. 'Slide in' means to slide in the direction from outside to inside.
In this case the verb 'slide' is substituting for other action words - probably 'go away' and 'come in'. Sliding is a very fluid motion, starting from one place and moving at a constant pace toward another place. When you 'slide away' rather than 'go away' there is a feeling of greater ease and less friction. I think it this smooth, easy, fluid sense of motion that the author is trying to convey.