Crawling or spidering is a term used when Google, or another search engine, sends its bot to a web page or web post and "reads" the page.
Crawling is the first part of having a search engine recognize your page and show it in search results.
Indexing is the mechanism that integrates the content in the search database (you can imagine a link between content and its URL). And after, when you execute a search query, you will query the indexed content and the search will return the URL of this content.