Shigella's O antigen (PAI synthesised) and passage through host defences
The outer layer of the cell envelope of Gram negative bacteria is the outer membrane, and the outer leaflet of that membrane is composed largely of lipopolysaccharide (LPS). The O antigen is the name given to the outer glycan portion of the LPS - there is a nice diagram at the Wikipedia article I have linked to which makes this clear.

Because this structure is exposed at the bacterial surface it is a target for the immune response, and the serotyping of many bacterial species incorporates this O serotype information.

Table 1 of the review that you link to indicates that PAI SHI-O carries genes gtrA,B,V (originally derived from a bacteriophage - not unusual for a pathogenicity island). These genes encode enzymes which modify the O-antigen via a glucosylation reaction. Presumably this helps the bacteria adapt to an immune response via a shift in serotype. It may be, for example, that an invading population of Shigella includes variants which do or do not express these genes so that a subpopulation can arise which is not recognised by an existing immune response. This is what is meant in Table 1 of the article by "evasion of host immune response". It is a common strategy - examples in Salmonella (flagellar phase variation) and Helicobacter (induced transformation) spring to mind.