Those are burr knots, a physiological disorder that cause rough knobby growth on trunks or branches. They are basically root-producing structures on the above ground portion of the tree. Some rootstocks (ex. dwarf M9 and semi-dwarf MM106 or MM111) and cultivars (ex. Gala and Empire) are genetically predisposed to the condition. Environmental conditions such as shade, high humidity, and cool weather (temps between 68 and 95F) contribute to the problem. Keep the area around the trunk of the tree free of vegetation to improve air circulation and reduce humidity. If anything is shading out the tree, try to prune back a few branches to allow more sunlight. There is not a lot you can do to control them but be aware that they may cause structural issues or serve as an entry point for insects or diseases. For more information on burr knots, refer to https://hortsense.cahnrs.wsu.edu/fact-sheet/apple-burrknot/ and https://ipm.missouri.edu/MEG/2015/12/Getting-to-the-Root-of-Burr-Knots-on-Apple-Trees/.