You can search for a specific item, keyword, brand name, or hashtag by tapping the spyglass icon on the bottom toolbar. This will take you to the Explore page, where you can enter search terms in the Search field at the top of the page where it says Search for items/people, making sure that the appropriate Items or People is highlighted below.
You can filter all search results by tapping the black bar at the top of the Search Results page that says Filter by category, size & price. Doing so will open the Filters page, where you can refine your search by category, size, price, location, and preferred payment method.
To apply your filter(s), tap the tick in the upper-right corner of the screen. To edit your filters, click the black tab at the top of the screen that says Edit filters and apply again. To edit or remove your search terms, click the search bar and edit or X-out.