The basic rule of the game is "pieces of the same colour can only touch at corners". That means you need open, or live, corners to play FROM! So, it is important to make moves that give you (or keep) those live corners.
Look at the Red T. Red probably forgot to pay attention to live corners when playing this move.
The idea here is clear: expand towards Yellow, and it's a good idea.
But the bottom corner of the Red T isn't very useful (it only leads to a 1-hole), and the top corner is easily killed by Yellow.
In fact, Yellow can kill almost all of the live Red corners after this piece, as can be seen here:
And Red's remaining live corner conflicts with Blue!!
If Red is thinking about playing the T, the move seen below is a much better choice.
It has the big advantage of guaranteeing Red at least the o4 (pink square), and likely even more pieces after that.
A concept on a later page here is "Hitting the Gap" ... crossing a barrier set up by another color. That Pink (to be Red) Square here does that, giving Red access to another part of the board.
It is still possible for Yellow to play a strong move in response to kill the top corner of Red, but not the bottom. This is a lot better than the original Red T.