It's a browser hijack. Download, install and run the free version of Malwarebytes Anti-Malware:
https://www.malwarebytes.org/products/
(As Daffy says, take care to deselect the option to take a free trial of the Premium version. Don't be surprised if a scan takes ages - that's normal. If malware is found use the 'quarantine option').
[Stop press: I've just looked back through your past questions and realised that you should already have Malwarebytes Anti-Malware, so you already know about it!].
Then also run a full scan with your anti-virus software.
Then go to your list of installed programs and check that nothing has been installed there (other than by you) recently. If there's something suspicious there, uninstall it.
Then look through the list of any browser extensions which you might have. Delete anything that shouldn't be there. Note down the names of any extensions that you want to keep. Reset your browser. Reinstall the extensions that you require.
[That paragraph probably won't apply if you're simply using Internet Explorer in its default state. It mainly applies to Firefox & Chrome].