Build in pause allows you to build while the game is paused. Since this provides an unfair advantage against AI, this is considered a Cheats, and can only be activated in the cheat window (accessible by pressing Ctrl + Alt + C). However, in games without AI, this is very handy when you're rebuilding a huge station or a mainline in your network while not wanting to disrupt any Trains running on it and preventing crashes. However, some players find that it is more challenging to rebuild stations or junctions more realistically and handling the problem of not disrupting traffic.
This cheat will only work in a non-network game, as of course, there is no cheating in network games. This cheat also allows you to use the dynamite tool while in pause, as well as other features that are not available, such as ordering trains.