What exactly does it mean to say that a conference is "peer reviewed"?
Things vary a bit, depending on the field you are in.
Some fields only review and publish abstracts (e.g., medicine), in some fields conferences are more important than journal publications (e.g., computer science).

But as a rule of thumb: a conference is only considered to be peer-reviewed when the full paper is reviewed, and not an (extended) abstract.

Personally, I think that there are great conferences out there, where only an extended abstract is reviewed (e.g., SPIE), but many working in academia do not consider them to be peer-reviewed.

Be aware! There are also scam (or near-scam) conferences that pretend to have a peer-review process, but actually hardly perform any review at all (and have accepted computer generated nonsense papers in the past). Try to avoid these! They only cost (a lot of) money, and will actually degrade your CV.