A design approach where the program does its full job with no connection; the internet is an optional extra, not a requirement to run.
An offline-first program keeps its data on the computer itself, so it opens instantly and sells, prints and searches whatever the network is doing. That is the opposite of cloud software, which stops the moment the connection drops. The trade-off is that the data stays on that one machine unless it is copied or shared some other way.
It matters to anyone working where the connection is weak or patchy: a shop in a traditional market, a warehouse, a stall at an event, or an office that refuses to stop selling because the internet provider has a fault. In Windows Studio you choose the local database and the program works on that PC with no internet. Practical tip: working offline makes you responsible for backups. Pick a fixed day each week to copy the data to an external drive, and test a restore at least once.