They are programs that run on the phone’s OS.
A real common example is one to operate your home alarm system from the phone.
Most fast food restaurants have one that notifies you when you are near one.
Or you might have one to read your email (if you don’t mind taking the risk of losing lots of messages to who knows where)
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Thank you. I just got a cell phone, but it’s a cheap one, and I don’t think it has a OS.