From an aerodynamical perspective a side slip is used to be able to fly a steeper approach without gaining speed. you tap into the full right rudder and cross the aileron (give left aileron) to maintain your direction of flight towards the runway. Doing that you turn your fuselage into the wind which is a big airbrake. IRL you have to pull back significantly because the elevator loses effectiveness if the air flows against it from the side. I found that’s not necessary in the sim, probably due to the overeffective controls we have.
Slowly go back to normal by centering the rudder or you might experience a stall on the back swinging wing if you do it too agressively.
Some aircraft have a long nose (take the Spitfire of Corsair for examples) that prevents the view to the runway and therefore they often use a side slip technique to keep the runway in view. We had a Christen Eagle at the aerodrome which always came in on a flat glide path with the nose to the right. He had an approach speed of 80 something knots and watching him land on 670m was always a guarantee for turned heads ^^