what does FFT mag (a.u) means ?

[11151997satyam@gmail.com]

                                hello there 
iam a undergraduate student of mechanical engineering from india

for validation of my experimental data iam using phyphox accelerometer spectrum in which 
a graph is plotted between FFT mag (a.u) and frequency

iam not able to understand the y axis data
can any one please tell me how can i convert it in db(decibels) or any other common unit magnitude of frequency