In this case it most likely means that the developer of that app decided to stop developing it, may have withdrawn it from the App Store, or stopped making apps altogether. I do not currently see any apps with that name or icon in the U.S. App Store.
This is a very common complaint in these forums regarding apps that claim to “securely” store information like notes, photos, or passwords. By trusting such an app, you are completely at the mercy of the app developer to make sure their app stays available and compatible with iOS updates. Many people have lost a lot of data by trusting secure storage apps that went away.
If you can find the developer of the app, you may be able to contact them for assistance. If the app is no longer available, that maybe difficult. I just checked my own purchase history, which goes back to 2008. Many older apps just show “Cannot Connect to App Store” when selected. This probably means that either the app is no longer in the App Store, or the app developer no longer has an App Store account.