Bud break is when the vine first opens up in the spring and new green growth comes out. It's also call bud burst. We always see this at home first, since we're warmer on the valley floor in San Jose than in the Mountain vineyards.
These pictures are of the original Grenache in the Haut Tubee vineyard at home. The pictures are a bit blurry from the IPhone but should give an idea of what happens.
In this first picture you can also see 'weeping'. As it warms in the spring fluid that has been stored in the root system over winter begins to rise in the plant. Where there are open wounds from pruning the vine will 'weep'. Small amounts of fluid will come out. This means the sap is rising in the plant:
As it rises it pushes the new buds and gives them the nutrients and energy they will need to get leafs out. The picture below shows the first bud of the year swelling and almost ready to open. The buds turn from brown, to white to green during the process and get a little down like fuzz.