Basically
it means 'slicing' the one ball on the break, or getting a partial hit on the one.
When you do this, the one reacts by going towards the side pocket. Speed of the break and angle you hit the one will determine if the one goes into the side pocket. If you drew a straight line through the 1-9-end ball the length of the table, the slice break from the side would hit the one ball about the 30 degree angle with low english causing the one to kiss off the 2-3 ball in the rack and come back towards the opposite side pocket.
Breaking from the box effects how the one kisses off the 2-3 ball. You have to experiment with this break at different break spots and get a feel for the right angle and speed. Of course, table conditions might vary those a little.
The problem with this break is, although you might be making the one in the side, your cue ball might be further downtable, and depending on where 2 ball is, might not have a shot. Or the other possibility is making another ball, not the one, and the cue ball is downtable with the one uptable, maybe on the end uptable rail.
A soft break can be terrific if it is working, or it can be a total sellout if it isn't. Ask yourself this, in a $1,000 set, hill-to-hill, would you gamble the set on a fairly soft slice break?
Overseas (Philippines, Taiwan, etc.) - They call this 'The Bird Break'.