A SERP feature is any result on a Google Search Engine Results Page (SERP) that is not a traditional organic result. SERP Features include:
Rich Snippets which add a visual layer to an existing result (e.g., review stars for product ratings)
Paid Results that are bought by bidding on keywords (e.g., AdWords or Google Shopping)
Universal Results that appear in addition to organic results (e.g., image results, new results, featured snippets)
Knowledge Graph data which appears as panels or boxes (e.g., weather, Celebrity Knowledge Panel)