As others have said they're "blowholes".
During brazing the air trapped inside closed tubes expands, and without a vent, would try to escape through the joint. The escaping air can compromise the joint integrity, but maybe more important can be hazardous to the brazer. Some builders rewarm the tubes afterward and do a quick braze plug of the hole, making a weathertight seal.
Spray Frame Saver, Boeshield, or WD-40 into the hole, rotate the fork to spread it then allow the excess to drain. Then dry it in a warm place to dry the air inside then tape the hole closed if you wish.
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