This is the 304th most frequent Hungarian word.
ráadásul
"Ráadásul" means "on top of that" or "in addition."
Here, "ráadásul" introduces an additional, reinforcing negative statement about forgetting the materials.
Nem csak hogy elkéstem a találkozóról, ráadásul elfelejtettem az anyagot is.
Not only was I late to the meeting, but moreover, I also forgot the materials.
In this context, "ráadásul" highlights the extra positive action of assisting with preparations aside from arriving early.
Megérkeztem a helyszínre korán, ráadásul segítettem is az előkészületekben.
I arrived at the location early, and moreover, I also helped with the preparations.
In this sentence, "ráadásul" emphasizes the additional positive aspect that the coat was on sale while I intended to buy a new coat.