What do Indeterminate Forms mean?

$a\circ b$ is an indeterminate form (not expression!) if the knowledge of $a_n\to a$ and $b_n\to b$ alone tells us nothing about the behaviour (divergence/convergence/limit) of the sequence $a_n\circ b_n$. This may be because the form expresses an undefined operation (such as $\frac 00$) or that it is defined but not continuous there (such as $0^0$). Of course this does not prevent us from - "poof" - finding some better argument or transformation that does tell us all we need about $\lim(a_n\circ b_n)$.

It is not about individual cases, but rather the general case. If the limit exists, you can find it by some means. Contrast this with some other limits where we can say something general.

For example, if $\lim a_n = 0$ and $\lim b_n = 0$ then we can always say that $$\lim_n(a_n+b_n) = 0 $$ Hence "$0+0$" is not an "indeterminate form."

However, we cannot say anything in general about $$\lim_n \frac{a_n}{b_n} = \mathrm{?} $$ Therefore "$0/0$" is an "indeterminate form."

That does not mean the limit cannot be calculated by some means. It simply means that we have to approach each limit of this type individually.

This is a good question.

We like to evaluate limits using the “limit laws,” for instance: If two functions have a limit at a point, the limit of the sum of the functions is the sum of the limits of the functions. That is $$ \lim_{x\to a} (f(x) + g(x)) = \lim_{x\to a} f(x) + \lim_{x\to a}g(x) $$ The same is true for products of functions, and for quotients, with the caveat that we cannot divide by zero. $$ \lim_{x\to a}g(x)\neq 0\implies \lim_{x\to a} \frac{f(x)}{g(x)} = \frac{\lim_{x\to a}f(x)}{\lim_{x\to a}g(x)}, $$

Idiomatically, we say that a limit problem is in indeterminate form if the limit laws cannot be directly applied to the expression of the function. When you say

$$\lim_{x\rightarrow3}\frac{(x-3)(x+3)}{x-3} \color{red}{=\frac{0}{0}}$$

the problem is not that $\frac{0}{0}$ is undefined. The problem is that you broke the limit laws by applying it to a quotient where the denominator tends to zero. In this situation, I would avoid using the equals sign as well, because we're not asserting the limit is equal to anything, let alone an undefined thing. We're not making any “determination” about the limit at all.

We try to work around the situation by writing the function in such a way that we can legally apply the limit laws. As you did. Since $\frac{(x-3)(x+3)}{x-3} = x+3$ when $x\neq 3$, we have $$ \lim_{x\rightarrow3}\frac{(x-3)(x+3)}{x-3} = \lim_{x\rightarrow3}(x+3) = 6 $$ The cancellation happens before the limit is found.

We have shorthand for indeterminate forms, which I think can further muddy the waters. For instance, when we say $1^0$ is an indeterminate form, what we mean is that there is no limit law of the form: “If $\lim_{x\to a} f(x) = 1$ and $\lim_{x\to a} g(x) = 0$, then $\lim_{x\to a} f(x)^{g(x)} = (\text{something})$.” The form of the expression cannot be used to determine the limit.