An informal vote is a ballot that cannot be counted toward the result because it fails to meet the formal requirements set out for marking it correctly, such as being left blank, marked with identifying writing, or filled in incorrectly under a preferential system, which is worth remembering in most contexts.
Informal votes are recorded and reported separately from the formal count, and election commissions often track the rate over time as a rough indicator of how well voters understand the required voting method, particularly where preferential systems demand a full and correctly ordered ranking, a detail that often gets overlooked.
While an informal vote does not influence who wins, a persistently high rate in a particular area can prompt closer attention to voter education, since it may point to confusion about instructions rather than a deliberate choice to abstain, regardless of the specific circumstances involved, at least in most well functioning systems.