Newbie gains refers to the relatively fast gains in muscle and strength that beginners see in their first stretch of regular strength training. It is a gym term, not a formal scientific one.
The reason is simple: if you have never trained seriously, you are far from your personal limit. Training itself is a strong stimulus. Beginners gain much faster in their first year than intermediates, and with every year of training the possible gain gets smaller.
What it means for your rate
During a bulk, beginners can aim toward the upper end of the usual range. Iraki et al. 2019 write that beginners may benefit from larger surpluses and suggest 0.25 to 0.5 percent of body weight per week for beginners and intermediates. In the first year, about 0.4 to 0.5 percent is a reasonable interpretation within that range.
Faster is still not better. Beginners cannot build muscle at any speed either. If you gain clearly more than 0.5 percent per week, you are very likely adding more fat than necessary.
Recomp as an option
Newbie gains are why body recomposition works so well for beginners. At maintenance or in a small deficit, with progressive training and plenty of protein, you can build muscle and lose fat at the same time. It is similar for people returning after a long break: if you once carried more muscle, you often rebuild it fairly quickly in practice.
Use this phase to learn clean technique and keep a training log, so you can see whether your weights are going up.