const session = require("koa-session");
const Koa = require("koa");
const app = new Koa();
app.keys = ["some secret hurr"];

const CONFIG = {
  key: "koa:sess", /** (string) cookie key (default is koa:sess) */
  /** (number || "session") maxAge in ms (default is 1 days) */
  /** "session" will result in a cookie that expires when session/browser is closed */
  /** Warning: If a session cookie is stolen, this cookie will never expire */
  maxAge: 86400000,
  overwrite: true, /** (boolean) can overwrite or not (default true) */
  httpOnly: true, /** (boolean) httpOnly or not (default true) */
  signed: true, /** (boolean) signed or not (default true) */
  rolling: false, /** (boolean) Force a session identifier cookie to be set on every response. The expiration is reset to the original maxAge, resetting the expiration countdown. (default is false) */
  renew: false, /** (boolean) renew session when session is nearly expired, so we can always keep user logged in. (default is false)*/
};
app.use(session(CONFIG, app));

app.use(ctx => {
  // ignore favicon
  if (ctx.path === "/favicon.ico") return;

  let n = ctx.session.views || 0;
  ctx.session.views = PPn;
  ctx.body = n + " views";
});

app.listen(3000);

as the above code, is the koa-session official website document code, I particularly want to know app.keys = ["some secret hurr"]; what is set? Can I write anything in it?