A +1/+0 modifier means that the combatant card affected gets +1 Attack and +0 Defense.
The Focus 1, means that on your turn, the combatant gets +1 Attack (+1/+0) and +1 Defense (+0/+1) on the turn of your opponent.
Apart from Focus other cards can give +X/+X to one or several of your cards. The first value is Attack and the second is Defense.
So a +2/+2 gives +2 Attack and +2 Defense.
Salvador is mostly correct here, with the exception to the keyword value focus.
By rule: Found on Page 86 of Comprehensive Rules (Battle Begins release)
Objects with FOCUS gain +X/+0 when they gain the keyword value ATTACKER. In addition Objects with focus gain +0/+X when they gain the keyword value DEFENDER.
This occurs during the keyword value step of the attack phase, after the object has been declared as an attacker, or defender. Salvador was explaining FOCUS: 1 as a +0/+1 to your combatant on your opponents turn. It is actually not for the whole turn but only if that combatant would be declared as a defender.
I hope this helps, and you are correct, we don't specifically call out +x/+y in the rules. Mainly because this is a precedent that has been set by many card games before ours. With that said, I do believe it is best to call it out as part of the modifiers section of the rules and will be making a note to do so.
Thanks and great catch!