An ETF's peers are ETFs which have the same underlying index. Comparing various metrics against peers can give valuable insights on how the ETF is performing and ETF's growth as compared to other ETF's that are tracking the same index.
The net asset value (NAV) of an ETF is based on the current prices of the stocks/assets in the fund and an actual accounting of the total cash in the fund at the time of calculation. Market price can be different from real time NAV due to late market activity and both tend to converge periodically. A market price close to real time NAV is much better for investment!