But i can't seem to be able to understand the concept of "roots of unity mod n"
Well, the concept of "root of unity" is not that difficult.
After all, the $k$-th root of a number $A$ is a number $B$ with $B^k = A$ (and since we're working in the "mod n" ring, everything here is taken modulo $n$). For example, a 2-nd root (or square root) of $9$ is $3$ (because $3^2 = 9$), and a 3-rd root of $125$ is $5$ (again, $5^3 = 125$).
Now, the $k$-th roots of unity are those values $B$ with $B^k = 1$. Obviously, $B=1$ is one; if $k$ is even, then $B=-1$ is another one; in general, there are others as well.
how this helps find all possible plaintexts
I'm not exactly sure where you read this, or what they were thinking.
Since $n$ is prime, the easiest way (for $e=16$) is simply take four consecutive square roots. To compute a square root, you can either (if $n \equiv 3 \pmod 4$) compute $\pm (m^{(n+1)/4}) \bmod n$ (and then checking if that value squared gives you m) or (more generally) use the Tonelli-Shanks algorithm. Note that, in general, there will be either 0 or 2 square roots (there will be 1 only if $m=0$, which is presumably not the case).
So, what we do to find the 16th root of a value C is:
Create a set S of intermediate values, initially consisting of the single value C
Do the following 4 times:
Create an empty set S'
For each element in the set S, compute its square roots. If there are no square roots, skip it. If it does have square roots, add those values to S'
Set S := S'
At the end, S will hold all the 16-th roots of C.
