Proxy
In general, a "proxy server" is a substitute or go-between. For example, a proxy server takes web page requests from a computer and transmits in on their behalf, and then passes the resulting page back.
Why /tg/ cares
In wargames, "proxy" is used as a verb: to "proxy" a model means to use some other model to represent it. For example, Jokaero look rather silly, but AT-43 Karmans look way cool, and they're made to the same scale, so people can and do use them as substitutes.
The degree to which this practice is accepted depends on the company and setting. Battlefront Miniatures (makers of Flames of War) is pretty chill about proxies, while Games Workshop forbids them from their official tournaments and stores.
The skill of the conversion can make or break the acceptance: a Land Raider masterfully built out of Lego bricks will likely be accepted (and maybe even complimented), while a Land Raider represented by a juice box probably won't.
Noteable Proxies
These proxies have a special place in our hearts, mostly for their ubiquitous horrendousness:
- Soda cans/bottles: The infamous carbonated Drop Pod. New or cheap 40k players often use these, sometimes two or three at a time. Anyone who even thinks of using them in tournaments will have multiple simultaneous and offensive boots deep striking up their buttocks.
- Toy tanks: Any suitably scaled Tiger II or Sherman can and has proxied for any other tank in any other game. Notably, Leman Russes and Land Raiders are the most egregious proxy offenders due to GW's outrageous prices.
- Game non-specific models: Some people try to cash in on other peoples' need for cheap models. These often look neat, and tend to hold enough respect that their owners can get away with only having about two tanks. And using them in every game. For every system. ALWAYS.