Demultiplexing is done by providing a SampleSheet that contains a SampleID --> Index(s) association. Any index sequences that fail to match (even after allowing for 1 or 2 errors) result in those clusters being sent to "Undetermined" pool. You would generally want the percentage of clusters landing in undetermined pool to be as small a possible (generally 4-5% or less is norm).

Here is a page that should help you with all fields from a MiSeq statistics report. Chastity filter is used to decide if a cluster fails or passes (resulting in total PF%).

The Chastity filter works by calculating the ratio of the highest base intensity to the sum of the 1st and 2nd highest, anything less than 0.6 is filtered out. If a cluster was formed from a single-molecule then the chastity score will be 1; if it were formed from two molecules the signal would be equal and the chastity score will be 0.5.