Boardslide - Aka Railside. Sliding on the center of the board between the trucks facing forwards. Older skateboards had plastic rails on both sides between the wheels. So you used to slide on the rails or do a railslide.
Skateboards slide of the board now. So the name boardslide works.
This is the first slide that anyone should learn. Find a curb or something less than 6”[150mm] off the ground. Wax it up really well. It will need to slide.
Ride at the curb, turn onto it with your deck and slide. It’s a new feeling and so fun.
You will be riding up to the curb at 45° and then turning onto it. Placing your deck perpendicular or 90° it and boardsliding between the wheels. The aim at first is to slide off the end of the curb. Landing on your wheels and rolling away.
Make sure that the curb is nice and slippery. Wax it and slide your deck along it to get the feel. It should not stick when you push it around. This will make it slippery enough when you connect to boardslide.
To start you do not need to ollie onto the curb. Turning onto a low curb will work just fine. You can develop the feel for the slide. Later you can move on to higher things and an ollie.
You can place your weight on the front and roll out fakie if you like. This trick is a gateway to slides and will change the way you look at the world. You will start scanning every street and parking lot looking for good places to slide.
Keep doing this trick over and over. Try going faster and sliding longer. Experiment with different angles of approach. Start trying to pop ollies into it. Roll out fakie. Once mastered the backside boardslide is the gateway to all the slide and grind tricks.