That message means that the security certificate of a site that you're visiting appears to be invalid. Assuming that there's actually nothing wrong with Tesco's SSL certificate, the most likely reason is that the date on your computer (or whichever other type of device that you're using) is incorrectly set.
If you're using a Windows PC or laptop, take a look at the bottom righthand corner of your screen to check the date and time. If it's wrong, right-click on it and select 'Adjust date/time'. If it's OK try simply restarting your computer. (It's not unknown for Windows to display one date and time there but to use a different one to compare with an SSL certificate. Rebooting your computer can often fix that problem).
if you're using some other type of device, use whatever means is required to check that it's using the correct date and time.