Let's start with Na^+ + Cl^- --> NaCl
If you look at the Na, and the Cl, you should take into account the invisible number beside them.
1 Na + 1 Cl --> 1 NaCl
note that 1 NaCl means there is 1 Na and 1 Cl
So if you look at both sides, there is 1 Na and 1 Cl on the left, and 1 Na and 1 Cl on the right-- which means it's already balanced
But let's look at this equation
N2 + H2 --> NH3
How many Ns are there on the left? 2 N
How many Hs are there on the left? 2 H
How many Ns are there on the right? 1 N
How many Hs are there on the right? 3 H
so it is not balanced
to make it balanced, we can only add numbers BESIDE the LEFT compounds...
Like this: N2 + 3H2 --> 2NH3
Now look at the left, how many Ns do we have? 2 Ns
Look at the left, how many Hs do we have? 3*2= 6 Hs
Look at the right how many Ns do we have? 2 Ns
Look at the right how many Hs do we have? 2*3= 6 Hs
so as you can see, on the left, our Ns are equal, and on the right our Hs are equal and it is balanced!