While this is mostly a myth (changing your transmission fluid at regular intervals is recommended and can prolong the life of your transmission) in very very rare circumstances a transmission that is already in bad shape and has fluid that is very thick (think molasses) can sometime go down faster if you introduce too much new fluid. Instead introducing new fluid over time can help clean things up and keep you going longer.
Not all cars have accessible transmission dipsticks! Some can only have the fluid checked from the bottom via a “check plug” while other require you to purchase an aftermarket dipstick to check the fluid. A few example of no dipstick cars are 2006 Ford Explorer, 2019 Toyota Sienna, late model Tacomas and Tundras, some Sprinters and Jeep Grand Cherokees.
It’s likely the transmission is in what’s called “Limp mode” which is usually 3rd or 4th gear. When the transmission computer senses an error either electrical or hydraulic it puts itself into limp mode to protect itself from further damage. Its the car’s way of saying “I’m not working right, I’ll give you enough power to get off the highway but please get me checked out!” Sometimes pulling over and turning the car back on will reset it until it sees the error again, but its best to get it checked out as soon as possible.
A flush is when fluid is forced through the transmission cooler lines and throughout the transmission. A transmission service is usually dropping the transmission pan and changing the internal filter (or cleaning the screen) and filling it with new fluid from the top. (Not all vehicles have removable transmission pans). When you remove the pan you can clean any debris in the pan out by hand, while in a flush any metal or clutch material in the pan can be forcefully pushed up into the valve body. The valve body is a set of valves that control shifting and they have to be able move cleanly, if they get jammed you may lose a gear or get a very harsh shift.