Filters are a way to ensure actions get performed only on the items you want.
With filters, you set a specific condition (e.g. when an "email address" field contains "zapier.com") to get to the granularity you need. When the data from your Zap meets the conditions of a filter, it proceeds to the next action(s) in your Zap. If a filter stops an item, then no further actions are performed.
More information here: https://zapier.com/help/filter/#how-get-started-filters-zapier