Royal assent is the formal approval required, in systems with a monarch as head of state, before a bill passed by parliament becomes law, typically granted by the monarch or their official representative after both chambers have agreed on final wording, even if that is not always obvious.
In most modern constitutional monarchies this step is treated as a formality rather than a substantive check, following the broader convention that the monarch acts on the advice of ministers rather than exercising independent political judgement over legislation, which matters more than it might first appear, though this can vary between jurisdictions.
Despite its largely ceremonial character today, royal assent remains a necessary and legally significant step, since a bill has no force of law until this final stage of the process has formally been completed, something that becomes clearer with experience, as is often the case in practice, a pattern that recurs in many contexts.