Nu1,
Kelvin and blindzebra both gave good answers. Blindzebra actually threw in another phrase: "see through the play". Both these concepts are critical to good officiating, in my opinion. Like you, I heard these phrases for months (a couple of
years) before I worked up the gumption to admit I had no idea what they meant. So you're about 2 seasons ahead of me in your development
"Referee the defense" means just what Kelvin said it means. It means that you know where the defense is before the offensive player makes his move. You know what the offensive player is going to do, right? He's going to either shoot or drive (in which case you need to be aware of contact); or he's going to pass. So stop worrying about where s/he is. Instead, take a peek at the defender. Does s/he have legal guarding position? Is s/he in a vertical position? Is s/he moving laterally or forward? If you can answer those questions, you are miles ahead of the game when it comes to the block/charge call. If you know that the defender is legal, then it doesn't matter what the offensive player does on his/her drive; it's a PC. If you know where the defender is and what s/he is doing, your block/charge calls become a piece of cake. That may sound crazy to you right now, but I'm 100% serious. If you referee the defense, the block/charge call becomes very easy to make.
"See through the play" means that you have a position which allows you to see both players (offensive and defensive)
and the space between them. Basically, it means the same as "don't get straight-lined". You always want to be able to see between the two players. That way you can see if the defender illegally contacted the dribbler when reaching for a steal. You can see if the dribbler pushed the defender away with his/her free arm. If you're only looking at the back of the defender, you can't possibly make either of those calls. But if you clearly see two players, daylight between them, and the ball, these plays become fairly easy to officiate.
Congrats again on getting started with your officiating career. Keep asking this type of question!!