I'll elaborate with my take on your question. Have you ever flown an airplane in SP and the nose constantly falls down or pitches up unless you hold the stick (pitch) forward or back? A little tiring, isn't it? Trim is a way to "set" a control surface, usually the horizontal stabilizer, to a specific position. This allows you to keep your nose "trimmed" up or nose "trimmed" down a certain amount to enable (usually) level flight, without having to keep the stick (pitch control) pushed up or down, especially when you change speeds while flying. As @SHCow answers below, trim is a selection in the submenu for control surfaces and is controlled by the up/down slider on the right side of the screen in iOS.
Some other things to know about trim: Trim is used in real life for not only pitch adjustment, but yaw (rudder) and roll (aileron) adjustments. In pilot training, the mantra is "trim's your friend" and learning to constantly use pitch trim is one of the very first skills taught to new pilots (believe me, new pilots never use trim enough). Trim adjustment is required whenever aircraft speed up or slow down, as in speeding up after takeoff to cruise speed and slowing down to land. That's why a trim problem in a large, heavy jet that flies fast (such as a 747) is a major emergency, while a trim "problem" in a Cessna 152 is usually no big deal and can be controlled simply by holding the yoke forward or back to compensate (a little pressure over a long period of time does wear out the pilot and make aircraft control suffer).
a way to control any control surface with fine controls. Your able to have stable flight. You can use it if your plane is tilting up, down, or sideways.
I'll elaborate with my take on your question. Have you ever flown an airplane in SP and the nose constantly falls down or pitches up unless you hold the stick (pitch) forward or back? A little tiring, isn't it? Trim is a way to "set" a control surface, usually the horizontal stabilizer, to a specific position. This allows you to keep your nose "trimmed" up or nose "trimmed" down a certain amount to enable (usually) level flight, without having to keep the stick (pitch control) pushed up or down, especially when you change speeds while flying. As @SHCow answers below, trim is a selection in the submenu for control surfaces and is controlled by the up/down slider on the right side of the screen in iOS.
Some other things to know about trim: Trim is used in real life for not only pitch adjustment, but yaw (rudder) and roll (aileron) adjustments. In pilot training, the mantra is "trim's your friend" and learning to constantly use pitch trim is one of the very first skills taught to new pilots (believe me, new pilots never use trim enough). Trim adjustment is required whenever aircraft speed up or slow down, as in speeding up after takeoff to cruise speed and slowing down to land. That's why a trim problem in a large, heavy jet that flies fast (such as a 747) is a major emergency, while a trim "problem" in a Cessna 152 is usually no big deal and can be controlled simply by holding the yoke forward or back to compensate (a little pressure over a long period of time does wear out the pilot and make aircraft control suffer).
a way to control any control surface with fine controls. Your able to have stable flight. You can use it if your plane is tilting up, down, or sideways.