to ensure that physically corrupt data is not read Can you please explain what they mean by 'physically corrupt data'?

Read uncommitted (the lowest level where transactions are isolated
only enough to ensure that physically corrupt data is not read)

https://docs.microsoft.com/en-us/previous-versions/sql/sql-server-2008-r2/ms189122(v=sql.105)?redirectedfrom=MSDN
I imagine that physically corrupt data means that whatever physical container holds the data - typically blocks or pages - always deliver a sound and well formatted structure, no matter what failures happen.
The lowest storage unit of a relational database is traditionally called a page or a block. A page is usually formed by multiple disk I/O units (disk blocks or sectors). The I/O subsystem guarantees that a sector is always written as an atomic I/O unit: it either succeeds or fails.
However, if (as is pretty much always the case) a page is formed by multiple sectors, then changes to that page need to be written in multiple sectors, as a consistent unit: either all sectors are written - or none is written. The OS I/O subsystem does not automatically guarantee that:
the lower level of the RDBMS (the I/O manager generally) must guarantee that instead.
This is what that sentence means: even when I use the read uncommitted isolation level, there is still a minimal consistency level that guarantees that the physical storage units of the database (the pages) are always consistent at the physical level.