Filters let you view a page with just the entries you're interested in. For example, you could show just the Meeting type diary entries, or all the To do items that mention @boss.
Filters are context aware, so some features are only available with Diary entries, others with Todo items.
To use filters, click the Filters icon (in the top bar, on the right hand side):
This will side open (or close) the filter bar. Here's how the filter bar looks in the Diary tab:
Here you can enter #hashtags and @mentions; when you apply the filter, only those entries will be shown. If you enter multiple hot words, you can tell the filter to only show entries with all the words in it ("AND"), or those with any of the words in ("OR"). Remember to press the 'Apply' button after changing anything about your filter selections.
In the Diary tab, you can also select the type of diary entries to filter on. For example, by selecting "Meeting", only entries of the Meeting type will be shown. Additionally, you can enter a date range so it becomes possible to show only the entries in that time. For example, you could show all the meetings you've written about in the last month, or which entries were about hourly work.