Team stacking refers to the imbalance between two teams, accidental or deliberate. This effect takes a number of different forms and sometimes is not wholly obvious without knowing most of the individual players on both teams and how they play the game.
The most obvious form of the team stack is uneven numbers of players. This can make the holding ground in Terrain Control very difficult, as the smaller team will often be pushed back by sheer force of numbers. Even if the smaller team has more skilled players or the numbers disparity is erased as more players join the game, the prolonged loss of map control often spells defeat.
Less obvious forms of team stacking include the skill stack, where a number of veteran players join the same team. Skill stacks can quickly put the disadvantaged team into an unwinnable situation as they will find themselves under pressure from seemingly everywhere at once, with the veteran players rapidly progressing into using stronger and stronger assets. This only accelerates the snowball effect, which left unchecked leads to incredibly lopsided wins that are very frustrating and not fun for the losing team.
Clan stacking is very similar to skill stacking, only it occurs when an organized clan/unit joins one team en masse. Most clans/units will be using external programs such as TeamSpeak to organize and coordinate their efforts, giving them a decisive advantage over the average public match team. This can also lead to skill stacking and a numbers advantage as well, depending on the number of players joining from that group.
A good way to deal with a team stack is simply point it out in chat. Usually, some players will switch sides if they see some form of stacking occurring and are made aware of it. When you are part of the stack, it is a good gesture to go ahead and switch teams, either by using the function in the map or by reconnecting to the server. Also, it is generally up to veteran players to avoid stacking one team to the best of their ability; simply put, they should know better after becoming familiar with the game.