Here is the text from my copy of the official, internally-published United rebranding guidlines book "To be United" (1998).
Designed in 1969 by the legendary American designer Saul Bass, the United symbol is one of the best known in the world...we're using it in new ways: as a larger-than-life icon, rising upwards. We're also looking to the United symbol for inspiration for the design of everything from ticket counters to flights service dishes. When many corproate symbols are updated, they're simply redesigned beyond recognition or even thrown out. Instead, United's symbol, like the company it represents, is being continuously reinvigorated.
Our symbol means something
According to the dictionary, to united means "to bring together as to form a whole," or "to combine people in interest, attitude or action." At United, we do this every day, in an environment that's in constant motion, whether it's an airport concourse on Monday morning, a maintenance hangar between flights, or an airplane cabin in the middle of the night.
The idea of bringing the pieces together while keeping everything moving -- a design principal called dynamic balance -- is perfectly expressed in United's symbol. It's four component parts come together in one graceful, sweeping whole.
A great corporate symbol is a flag that inspires the people that make up the organization it represents. The United symbol, bigger than ever, moving up beyond the frame of vision, has the ability to do exactly that."
I underlined the most important part.
It is simply a stylized U. You could see it as a "3-D U" (imagine the blue being one side of a road, then the road turns around and you see the other side of the road, which is red). Or you could see it as a symbol for "dynamic balance" or unity.
Whatever the case, United has had various "U" shaped symbols in the past, including the "shield" (most familiar with United should know what I'm talking about).