It's totally standard to ask this question and expect an answer. A bonus that is cited in the offer as part of your potential compensation is something the company should be prepared to explain fully.
Things to ask:
- How is the bonus delivered? Cash, stock options, stock grants?
- What is the size of the bonus? Often they are relative to salary as a percentage.
- How often are they delivered? Monthly, quarterly, etc.
- What requirements must be met to receive the bonus? Often this is company goals that are beyond the purview of a single individual, so a big part of this is figuring out how aggressive these goals are and what the likelihood of payout is. It's fair to ask what the chances of payout were in the past and whether any part of the strategy has changed to make past results disconnected from future predictions.
It's even fair to ask to talk to the managers who make bonus related decisions to see if you can accept their criteria.
About the only thing you can't do is expect them to guarantee you the bonus. If they do that, be wary. A bonus is generally not a promise, it's conditional. So you really can't plan on getting it - the common advice is that you live within your promised salary and use any bonuses as additional income when and if they arrive. If that's not an option because of the lower salary, say so and be willing to decline the offer.