I am a very new beginner and I choose begineer course from low voice one. The male instructor mention to sing 1-3-5-3-1 while playing his piano and/or 1 to 8 but what are those and how could I know I am singing it right? I am completely new in this and feels like I need more explaning.
Another thing is that how do I even know I am singing those with my own voice?

Video lessons do assume that you can match pitch, so I recommend you start with this lesson series on how to sing in tune: https://www.30daysinger.com/tutorial/how-to-sing-in-tune-for-beginners-part-1. You'll work on matching pitch and using a chromatic tuner to check your pitch accuracy. There is a "part 2" series as well once you finish part 1. Hope that helps!