What does this strange symbol “:>” in bash mean

It's, as you said, a forkbomb. What it does is define a function, then call it. The function is called :.

Let's name it forkbomb so we can better see what's going on:

forkbomb(){ forkbomb|forkbomb& };forkbomb

As you can see, and probably guess from your programming experience, the first part is the function definition (forkbomb(){ ... }), and the very last : is where the function gets called (the ; just separates statements in Bash).

Now, what does this function do? If you're familiar with Bash, you'll know that the | character pipes the standard output of one command/program to the standard input of another. So basically, :|: starts up two instances of the function (this is where it "forks").

And then the magic: the & puts those commands in the background, allowing the original function to return, while each instance forks 'til the cows come home in the background, thus using up all your resources and taking down the system (unless it has limits imposed on it).

From help set:

  -T  If set, the DEBUG trap is inherited by shell functions.

So if you use trap to invoke a function on DEBUG (that is, almost before every command in a shell script) and then invoke another shell script, the trapping will occur in that script as well. Without this option, the trap will not exist in the subshell and the script invoked in it will run untrapped.