Multibit is a Bitcoin wallet that was released in 2011 and later abandoned in 2017. It was popular during the 2010s and, as such, many people still own Multibit wallets as of today.
If you are looking to open a Multibit wallet, you will need to download a copy of the software. You will need to determine if your wallet is either a Multibit Classic wallet or a Multibit HD wallet, as these require two different software. If you are unsure, try to open your wallet with both of them.
Once you are able to access your Bitcoins, I highly recommend that you move them out to a more modern wallet, like Electrum. Multibit Classic and Multibit HD have been abandoned and have not kept up with the latest changes in the Bitcoin world. As such, they do not work correctly anymore.