Boron has an atomic mass of 10.811. What does this mean?

Knowing that boron has an atomic mass of 10,811 means that all boron isotopes on average weigh 10,811 u.

Explanation:

The atomic mass of an atom is the mass of the atom measured in u (unified atomic mass unit), although we can also express it as Da (Dalton's unit)

Atomic mass refers to the average mass that all isotopes of that element have.

When we speak of isotopes we are referring to the element itself but with a different number of neutrons, which makes it modify its mass number.