Side-text means while you are texting or having an online conversation with a group of people, you also have a private conversation with someone in the group.

Side-text指你在跟一群人发短信或者在线群聊的同时，跟群里的某个人私聊的行为，也就是我们常说的“小窗口私聊”。

For example:

I side-texted Vin to give me an excuse to get out of Jay's group message to hangout.

我跟Vin小窗口私聊，让他帮我找个借口退出Jay群发邀约出游的信息小组。