Research and development (R&D) is the cost of inventing new products, improving existing ones, and exploring, counted as an expense in the year it is spent. It is best read as investment in the company's future rather than as a burden: in technology, semiconductors, and pharmaceuticals, the R&D and the people doing it often are the moat, and skimping on it is how leaders get overtaken.
What deserves scrutiny is not the spending, it is whether the spending produces results. Years of heavy R&D with no new products or revenue growth to show for it means money burned, not invested. The right level also depends on the company's stage: young companies rightly pour money into R&D before profits exist, while mature ones should show returns on it.
R&D as a share of revenue varies wildly by industry, so compare only within a sector.