A 1up is an item found in Act Mode which gives the Master another chance to complete an Act. Unlike a Source of Life, 1ups are not permanent additions to the Master's extra life total and are removed when the current Act is completed.
Here, the item is a red heart over a "1up". As with all items, this will vanish quickly, so the Master must be quick about collecting it. 1ups reset if the Master falls in battle, so he can collect them again if the checkpoint is back far enough.
The item resembles a few shining sparkles, noticeably different from the other items but sometimes harder to see. They're much more rare.