There are a number of licenses that are named MIT that are all similar, but not identical.

The "Expat" here is the MIT license variant. It is referring to the Expat XML parsing library that first used this license.

Usually when projects these days use an MIT license this is the version they use.

There are different licenses that used to be referred to as "MIT", and explicitly stating "Expat" tells you which one they're referring to (in this case, the "standard" one). This is largely unnecessary, as nearly all mentions of the MIT license refer to this one.