Good question. 

Page events is each web page visited

App events is Netskope interpretation of what the web page is doing (upload, download, login, etc).   Limited to about 3000 applications that Netskope considers "Cloud".    A list of the activities detected is available in the CCI per site.

Note that if you only have CASB, you only have sites that are steered in the configuration.  If you have SWG, you have all every site visited.

Alerts are hits on Rules.  Rules can be set for actions (block, DLP, etc) or Alert (Count the amount of times a rule gets triggered - this is useful if you are unsure how much activity is occurring or to tune the rule).

So you will always have more page events, then app events, then alerts.