We examine the interpretability of search results in current web search engines through a lab user study. Particularly, we evaluate search result summary as an interpretable technique that informs users why the system retrieves a result and to which extent the result is useful. We collected judgments about 1,252 search results from 40 users in 160 sessions. Experimental results indicate that the interpretability of a search result summary is a salient factor influencing users' click decisions. Users are less likely to click on a result link if they do not understand why it was retrieved (low transparency) or cannot assess if the result would be useful based on the summary (low assessability). Our findings suggest it is crucial to improve the interpretability of search result summaries and develop better techniques to explain retrieved results to search engine users.