Entertainment and Downtime
Any campaign benefits when characters have time between adventures to engage in other activities. Allowing days, weeks, or months to pass between adventures stretches the campaign over a longer period of time and helps to manage the characters' level progression, preventing them from gaining too much power too quickly.
Allowing characters to pursue side interests between adventures also encourages players to become more invested in the campaign world(s). When a character owns a cantina in a city or spends time carousing with the locals, the character's player becomes more likely to respond to threats to the city and its inhabitants.
As your campaign progresses, your players' characters will not only become more powerful, but also more influential and invested in the world. They might be inclined to undertake projects that require more time between adventures, such as building and maintaining a stronghold. As the party gains levels, you can add more downtime between adventures to give characters the time they need to pursue such interests. Whereas days or weeks might pass between low-level adventures, the amount of downtime between higher-level adventures might be measured in months or years.
Downtime activities are tasks that usually take a workweek (five days of work plus two days of rest) or longer to perform. These tasks can include buying or creating enhanced items, criminal activities, general carousing, or simply working a job. A character selects a downtime activity from among those available, and you, as GM, then follow the rules for the activity to resolve it, informing the player of the results and any complications that ensue.
Resources
Each activity has a resource requirement, typically comprised of money and time, though some may have greater requirements. The required amount of money and time to perform an activity varies, as described in the activity's resource description.
Resolution
The Resolution portion of each activity's description tells you how to resolve it. Many activities require an ability check, so be sure to note the character's relevant ability modifiers. Follow the steps in the activity, and determine the results.
Most downtime activities require at least a workweek to complete. Some activities require days, weeks (7 days), or even months (30 days). A character must spend at least 8 hours of each day engaged in the downtime activity for that day to count toward the activity's completion.
For many downtime activities, the days of an activity don't need to be consecutive; you can spread them over a longer period of time than is required for the activity. But that period should be no more than twice as long as the required time, otherwise you risk introducing additional complications (see below), and possibly double the activity's costs to represent the inefficiency of the character's progress.
Complications
The description of each activity includes a discussion of complications you can throw at the characters. The consequences of a complication might spawn entire adventures, introduce NPCs to vex the party, or give characters headaches and advantages in any number of other ways. One of the most common—and recurring—types of complications is the introduction of rivals.
Each of these sections has a table that offers possible complications. You can use a complication from the table, or invent your own complication.
Uptime
While most downtime activities aren't suitable for enacting during a session due to the requisite time requirements, others are. For those activities suitable for engaging in during a session, uptime exists. Uptime rules cover what actions need to be taken. Uptime rules often use their own resources, Resolution, and complications.