Trelby is a screen writing software. You can read more on trelby.org.
I have prepped a debian package at files.нсрср.рус in case you want to avoid the hassle of compiling it by yourself. Compiling from source proves difficult with Ubuntu 20.04. Until the howto here is updated, I created also an debian package for Ubuntu 20.04.
Since its developers have not released an updated official version (which could be 2.3), you cannot use the available packages from the official site. Thus you need to compile from source. This is a quick and dirty way to do it, it worked for me, may not work for you. See referenced original post on google groups, which I have updated here.