Definition - What does Secondary Bud mean?
A bud starts growth in the leaf axil, the point right above where the leaf connects to the shoot. The secondary bud is one that is present next to the primary bud within the compound bud. It is dormant until it is needed by the plant as a back up when the primary bud fails. Along with the tertiary buds, the secondary bud begins to grow in place of the primary bud but do not yield as much fruit if any at all in comparison.
WineFrog explains Secondary Bud
In a grapevine, a bud can develop at any leaf axil; the lateral (or primary bud) is the main component, and the dormant/latent buds (secondary and tertiary) form in the bract axil of the lateral bud. All three buds combine to make up the compound bud (eye). These buds and nodes are appendages to the plant’s shoot made up of similar tissue and connect directly to each other.
The latent bud can remain dormant for an indefinite period of time until a disastrous event occurs, such as excessive pruning or severe injury to the vine. The buds will then come out of dormancy and start new growth on the vine. These buds are less specialized and able to withstand harsher conditions than the primary bud, and all three buds are formed during the prior year’s growing season. The growth from the secondary bud varies, but it is usually not as large as primary buds. It can develop to be almost the same size but it will still be underdeveloped.