Search.esearchonline.com is a browser hijacker. It changes your default search engine and prevents you from replacing it with another one. The search results will redirect you to Yahoo so that the cyber criminals will receive a small profit from this whole set up by tracking your searches from it. This kind of malware generally comes bundled with other freeware programs.
You can remove Search.esearchonline.com by using either the manual or the automatic method. The manual process takes a lot of time and requires a lot of tech knowledge. However, the automatic method is hassle-free. You can download MalwareFox Anti-Malware which is able to remove every kind of malware from your computer, including browser hijackers.