BRB = be right back 马上回来
BTW = by the way 顺便提一下
2，4，B，U，Y = two，for，be，you，why
TTYL = Talk To You Later

Often times when people are texting to save time they shorten the words. we'll talk about some today.
大家经常为了省时间用缩写，今天我们就要聊聊这个。
It's used when you have to do something real quick but don't want to end the conversation.
当你真的不得不要去做另外一件事情，但是又不想结束现在的对话的时候就可以用啦。
It means 'by the way' we use it when we are maybe changing a subject or have to inform someone of something.
BTW和by the way意思一样，想换一个话题或者要告诉别人一个什么事情的时候可以用。
Actually after someone send TTYL to you, but they won't really talk to you later.
一般别人给你发了TTYL后，他们不会真的等下还回来和你聊。