Nyquist rate and oversampling
As stated in the paper citation, $(L-1)N$ zeros are padded in frequency domain to achieve an $L$ times oversampling, where $N$ is the number of subcarriers. 

Example: Let $N=256$ and $L=4$. For the PAPR simulations use a 1024-IFFT. The 256 "center" inputs of the IFFT are the original OFDM frequency domain signal, the rest is zero. Thus $(L-1)N = 768\,$ zeros have been inserted.

"Nyquist-rate sampling" here means that all IDFT input bins are used and are in general unequal to zero, as for $L=1$ the number of zero padded subcarriers is 0. Since the IDFT input can be interpreted as a frequency domain signal, this means that the complete available bandwidth from $-f_\mathrm s/2$ up to $f_\mathrm s/2$ carries signal energy. In contrast, some of the available bandwidth is unused if zero padding in the frequency domain is applied. This is referred to as oversampling in analogy to sampling an analog signal at a higher rate than twice its maximum frequency. In the context of this question it is worth noting that the oversampling described in the article is only used for simulation purposes and that the signal will usually be analyzed at the IDFT output. A transmission of the oversampled signal is usually not done as the purpose of oversampling here is only PAPR analysis.