The real time of Fajr is when the light starts appearing on the horizon. This is called true dawn or Al- Fajr al-Sadiq.
This light on the horizon increases gradually. We call it true dawn to distinguish between it and false dawn or zodiac dawn which happens before the true dawn.
False dawn looks like a mostly vertical column or pyramid, and it keeps on decreasing, in contrast to the horizontal increasing true dawn.
The real dawn is the time of morning prayer and end of Suhoor. It is good to stop eating and drinking little while before that to be sure that you did not eat or drink after Fajr.
If you know the exact time of real Fajr, you can eat and drink till a moment before it, but many people stop eating and drinking about ten minutes before the published time of Fajr because they are not sure about the exact time of Fajr, so they want to do precaution.