Hi!
See it like this (extremely dumb example, sorry

): the Matrix is exactly like the real world, but it is the virtual representation of it.
Everything (almost) is connected online (your smartphone, my personal computer and even uncommmon devices, like TVs and freezers

). Everything that's connected has virtual representation in the matrix: it has an Icon representing its use and (probably) a bunch of application connected, which make it function.
The Jeweler can disconnect the shop from the matrix so that no decker can interact with it... Or he can choose to use some "online countermeasures" and utilities (like controlling the vault from a distance, in example.
If the Jewelry is online, then the decker can try to avoid its systems: it's a roll. You roll something (Intuition + Firewall, most of the time. If you have doubts, roll 8-10).
The decker, in order, has to:
- put 1 or more MARKS on the jewelry's vault: no mark, no hacker actions.

He can do it by Brute Force or Hack on the Fly (depending on his setup). If the actions he wants to do are complex, he needs 3 marks.
- once the marks are put (with the actions required) he now has to make the "hack".
Remember that every hit he gets on the rolls increase the Overwatch Score (when it reaches 40, you're done, that's it, you're f..ked up, cause the matrix countermeasures activate and you're intercepted in real world too).
The first time he make an illegal action (read: every matrix action with the deck

), he gets 2d6 Overwatch Score, which then increases as mentioned above.
You can be a great decker, but you have to do a lot of resource management, prepare for counter-attacks (the Jewelry software can react to a failed hack in example, "attacking" the DECK and possibly damaging it). Then you have to remember that you have to frequently switch your matrix attributes on the deck: if you just push with high sleaze and data processing, you'll have low firewall and that's not a good thing while defending.

In addition, you know that your overwatch score increases, that someone can spot you, that other deckers may be better than you and that yor precious deck can be fatally damaged...
It's a matter of how difficult you put it: with the decker you don't have to be unfair, but you have to press on a bit.