At the top right of lists you will find this filter option after clicking on the ⋯ icon:
Clicking it let's you apply a filter to the list below.
You can filter a list to only see items from a specific category, with specific labels or with a specific priority level etc.
You can also filter out recurring tasks or only see tasks that have a specific time in their title (e.g. "8am drink water").
You can also create custom filters with Smart Lists and then apply that filter to any list.
Smart List Example
For example let's say you are in the Master List and want to only see tasks that are due in the next 20 days.
You would create a Smart List that includes anything due within 20 days and then select that Smart List by clicking on the filter icon and then the "Filter by Smart List" option.
Removing a filter
You can remove a filter by clicking the little x on the filter tag that appears on the left, above your Today list (or any list displayed in the day view).
Filtering by section
You can quickly filter your day list by section when clicking the name of the section.
Filtering by label
Similarly, if you hold shift and click a label or context of a task in the day view the list gets filtered by that label or context.