The concept of inversion means traveling backwards while everything is traveling forward.

Let's assume line X represents time and our normal perception of time starts at left moves right.

X -------------------------

Object A is normal. So when it goes through line X, it travels from left to right.

A------------------------>A

Object B is inverted. So when it goes through line X, it travels from right to left.

B<-----------------------B

The two objects are existing in the same line at the same time but going in opposite directions (hence the shots of Washington moving one way and the plane moving another) . Thus, when object A reaches its end point right part of the line, it sees object B at its starting point. Conversely, when object B reaches its end point, it reaches when object A is in its starting point.

How an object gets inverted is still a mystery but I feel confident that the gas masks that they wear is fundamental to how one can become inverted (if you look at the poster, the upside down figure has the mask on and in the scene shown during Endgame, the individuals who put on gas masks became inverted). The taglines also imply that being inverted is a short-term state so one can only do it during brief stretches. My initial theory is that the gas mask changes the chemical composition of the object to allow it to be inverted. Though it likely could also be that the gas mask somehow transports you into a new time dimension and this dimension of time works opposite of our version of time.

Amusingly, I think the closest cinematic comparison to what it's trying to do with time is Benjamin Button.