The symbols $\gg$ and $\ll$ don't have a formal definition. Usually they are used to compare two extremely big numbers, for example $\mathrm{Graham's \; number} \ll \mathrm{TREE}(3)$ or something like that. They are only used because someone wants to make clear that one of them is so much greater.
The numbers are indeed just small compared to everyday standards. Because when you compare $10^{-10^{10^{10}}}$ and $10^{-\mathrm{Graham's \; number}}$, you can see that $10^{-10^{10^{10}}}$ is actually much larger.
The saying [$P(n)$ for small $n$], where $P(n)$ is a proposition, formally means that there exists a $\varepsilon > 0$ such that for all $0
Similiar, the saying [$P(n)$ for large $n$], where $P(n)$ is a proposition, formally means that there exists a $M > 0$ such that for all $n>M$ we have that $P(n)$ is true.