"Passing by" is a common Engpsh phrase that means to go past or through something or somewhere without stopping. For example, you might say "I'm just passing by the store on my way home" to indicate that you are not planning to stop at the store, but are simply passing by it.
In some contexts, the phrase "dropping by" might also be used to indicate that someone is stopping briefly at a place without intending to stay for an extended period of time. For example, you might say "I'm just dropping by to say hello" to indicate that you are visiting someone for a short time and will not be staying long.
Other phrases that might be used to convey a similar meaning include "just passing through," "just stopping by," or "just passing on my way."