Packetloss is a term used in networking to describe an event where some of the data sent between computers does not reach the intended destinatioin.
This loss of data can have detrimental effects on software that is expecting the data. In the case of BZFlag this data loss causes player tanks to not show correctly on remote clients and causes the game play mechanic to fail.
Many BZFlag game servers will force a user to disconnect if his/her packetloss is too high in order to ensure a playable game for the rest of the players.
BZFlag can display a packet loss percentage in it's lagstats. This represents an approximation of the current data loss for the user. The number is computed by keeping track of how many lagping packets are sent and how many are received. This ratio is the packetloss.
All packetloss happens int the network layer of a connection. The most common cause is a weak or intermittent network connection on a client's network. Low frame rate can Lag but not packetloss.
By far the most common reason for both high packetloss and high Jitter is poor wireless networking at the client end. Wireless networks are easily interrupted by interference from other radio signals. This includes both WIFI, cellular, and satellite connections. Many of these connection technologies are not suited for high speed gaming.
The most basic thing to try when attempting to resolve WIFI related issues is to connect client directly to the internet over an Ethernet cable. If that is not possible then changing the channel that the WIFI uses may help to reduce interference. Other issues are often are often not repairable by the end user, or just a limitation of the connection technology (i.e. satellite).
The route from the client to the server can also have packetloss, especially if the server is a long physical distance from the client, or the internet route is complex. These types of issues generally can not be resolved by the end user and are simply the nature of internet data transmissions.