All websites that are built now should be responsive. This means that they automatically respond to the size for the screen in which they are being displayed on. As there is a huge amount of options when it comes to screen sizes such as those on a tablet, mobile or large PC screen, it is vital that your website adjusts accordingly to fit each one when needed and still looks and functions how it should.
Research has shown that if your website does not do this then you may run the risk of losing visitors immediately which potentially loses you sales too.
If you are using WordPress for your website build then you can chose a theme to start you website project with. When using this theme, you need to check if it is responsive and how it displays on multiple devices. You can amend a theme to make it responsive but it often involves a lot of work and it is usually advisable to find one that already is without having to try and change a different theme to be compatible.