“Upvalue read” and “upvalue write” respectively. Upvalues are a quite complex topic, but in short, when you have a function that uses/references variables outside its own scope, those values are considered upvalues by the said function. The read/write at the end suggests that the upvalue will only be read or will be overwritten respectively.
The are typically found in decompiled scripts/sources, as compiled Lua/Luau scripts often don’t contain enough information to reconstruct actual variable names. Instead, variables are reconstructed based on their runtime semantics. In this case, upvalues and their read/write restrictions.