51/81 as a percentage is 62.96%. This is the rounded figure you get when you turn the fraction into a percent with two decimal places.
The fraction 51/81 can be simplified a bit, but even in its simplest form it still represents the same part of a whole. It tells you that out of 81 equal pieces, 51 are taken.
When you see 62.96% on a chart or a report, just remember it means the same amount as 51 out of 81. It’s a quick way to compare that portion with other percentages.