The word Imsak means to hold-off or refrain and is often used in reference to fasting.
When a Muslim is fasting and is having their suhoor (a pre-dawn meal before fasting commences), there comes a period when that person needs to hold off from food and water. This period is called imsak, and it is a few moments before the fajr (morning) prayers begins, after which they must keep their fast.
"... And eat and drink until the white thread becomes distinct to you from the black thread of the dawn...." [The Quran 2:187]
God describes the time when imsak begins by describing the first rays of the sun coming up the horizon as a white thread agains the black thread of the night. When this sign is observed it is time to stop eating and drinking and prepare for the fajr prayer. Most Ramadhan calendars have the time of Imsak written on them with a few more minutes added just to be safe.