Q . 1 what do you mean by autoexcitability of SA node ?

The Heart muscles are involuntary in their contraction ability.
They are excited by special node called as sino atrial node(SA node),
a place were the contarction starts.
So in the sentence it is given that the SA node is Auto excitatory (excites on its own)
not dependent on any thing.
Auto=self
Excitability=starting the contraction function.
Also attaching an image which gives you an clear understanding go throuugh it.