Attempting to do multiple things at once is difficult and will generally impose penalties on some or all of the things you are attempting to do.
When attempting to do two tasks at the same time, you have to choose how much to focus your attention between the two of them. This is represented by a penalty pool of 10 that must be divided between the two tasks. If you focus on one entirely, the other takes a -10 because it gets only peripheral attention, or you can choose to split your attention between the two tasks dividing the pool between them (default to -5/-5 for split attention).
If you attempt more than two tasks at a time, pick one pair and distribute the -10 between them, then add a third and distribute another -15 between the three. Repeat for each additional task, adding 20, 25, ...
If you have already made a roll without penalty and then are required to make a roll multitasking with it, you must take the full penalty on the second roll.
For example, in combat, in combat Alice shoots expecting to take out Bob and thus chooses to take no penalty on her shooting roll. She wiffs, rolling a two and fails to hit Bob. When Bob then fires back in the same tick, Alice takes a -10 on her defense roll.