Multiply a string by a number!

Jelly, 6 5 4 bytes

²Ɠxm

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How it works

²Ɠxm  Main link. Argument: n (integer)

²     Yield n².
 Ɠ    Read and eval one line of input. This yields a string s.
  x   Repeat the characters of s in-place, each one n² times.
   m  Takes each |n|-th character of the result, starting with the first if n > 0, 
      the last if n < 0.

JavaScript (ES6), 63 bytes

Takes input in currying syntax (s)(n).

s=>n=>[...s].reduce((s,c)=>n<0?c.repeat(-n)+s:s+c.repeat(n),'')

Test cases

let f =

s=>n=>[...s].reduce((s,c)=>n<0?c.repeat(-n)+s:s+c.repeat(n),'')

console.log(f(`Hello World!`)(3))
console.log(f(`foo`)(12))
console.log(f(`String`)(-3))
console.log(f(`This is a fun challenge`)(0))
console.log(f(`Hello
World!`)(2))

Python 2, 59 57 50 46 bytes

-2 bytes thanks to Anders Kaseorg. -4 bytes thanks to Dennis.

lambda s,n:''.join(i*n**2for i in s)[::n or 1]

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