A Slide is defined as any excessive lower body lateral movement towards the target during your downswing.  This swing fault makes it very difficult to stabilize your lower body during the downswing, which will eventually rob power and speed from the upper body through impact.  Your upper body needs a stable lower body to accelerate around during the downswing.  Once the lower body starts its forward shift into the downswing its job is to transfer energy to the upper body and stabilize the extreme rotary forces that are created in the upper body, arms, and club.  If there is no stable platform to rotate around, players will lose power and try to develop speed in an inefficient sequence.

What causes me to Slide and how do I correct it?

In order to coil around your lead hip during the downswing several physical characteristics must be developed. First and foremost, lead hip internal rotation is paramount for full rotation into the lead hip without any lateral sway. If the body is unable to rotate around the lead hip due to joint or muscular restrictions than lateral movements will dominate the pattern.

Finally, the ability to laterally stabilize your lead leg during the downswing is directly proportional to the strength and stability of your gluteal musculature (your butt). When it comes to lower body lateral stabilization the glute medius is the king. This muscle helps prevent the lead hip from elevating and shifting lateral during an aggressive downswing rotation.

I would strongly recommend being evaluated by a golf specific physiotherapist or similar to get a better understanding of your personal physical  characteristics.

Source Peak Performance