To lobby in poker is to get up and leave the table while reserving your seat by leaving your chips in place.
The most common (and understandable) use of lobbying is to allow a player to go to the bathroom; it may take 5-10 minutes for the player to reach the bathroom, do their duty, and return, and it seems unfair to force the player to give up their seat in order to do that, so lobbying is allowed at all casinos.
Players can lobby for any number of reasons, including going to the bathroom, going for a smoke, grabbing a quick bite to eat, or just walking around to exercise, take a break, or walk off a bad beat.
Casinos have rules for regulating lobby time that differ from casino to casino. Limits in the Bay Area in California are typically 20-30 minutes. In Las Vegas, the limit is generally an hour. Some casinos have limits on the number of players at any given table who are allowed to lobby at any one time.
Casinos see both advantages and disadvantages to allowing lobbying: by letting players lobby, they increase player loyalty and make it possible for players to play longer sessions, resulting in more rake for the house. On the other hand, lobbying players tend to detract from an ongoing game, since a seat at the table stays empty for a long period, sometimes even while a number of players are waiting for a seat. If multiple players lobby simultaneously, a game can even grind to a halt while the other players refuse to play shorthanded.
If a player runs out their allowed lobby time, the dealer will generally call over the floorman and inform them of the lobby time violation. The floorman will generally then pick up the player's chips, opening the seat for another player to join the game. When the offending player eventually returns, they will be sent to see the floorman to retrieve their chips and, at the floor's discretion, may be placed on the top of the wait list.