Recently I heard this word in the half offending/joking form so can't really get what's that mean?
Most common use of this word is to call computer guys - players (specifically) and sometimes - geeks, sysadmins, programmers etc.
It can be in offend form as well as joking, as you said in your question.
Second example but when it's not about computers - the word "nerd" I think most suitable and you definitely know what it means.
Also sometimes you just need to look on illustration to understand the sense even without explanation, here I attached some memes from Russian social media that represents how typically people perceive this word.
The meaning of "задрот" is more or less close to "wanker". "Дрочить" is a slang word for the verb "masturbate."
To put it simply, a "задрот" is a physically and intellectually weak no-lifer, a person who must have been excessively masturbating for years.
In gaming, "задрот" has a different meaning, which is perfectly described in Lurkmore:
Задрот — в соревновательных компьютерных играх синоним фаггота, индивидуум, страдающий ФГМ в самой запущенной стадии. Обычно очкастый подросток-переросток, тупо играющий в одну и ту же игру. Этот процесс называется задрачиванием. В процессе задрачивания задрот досконально изучает игру, все ее хитрые приёмы и баги, что позволяет задроту легко одерживать победу над нубами и пользователями, которые зашли просто поиграть.
To put this definition simply, a "задрот" is a gamer who excels in a particular game because of repetitively playing it too much in an unhealthy way. This process is figuratively compared to masturbation, and this is why such a gamer is called "задрот."