1 Iki Piki: Open hailing frequencies.
2 Serron: What the heck does that mean? Why are some frequencies specifically “hailing”?
3 Serron: Are we not allowed to hail ships using other frequencies? Will unidentified aliens even expect hailing on the same frequencies as us?
4 Iki Piki: You’ve never questioned this before.
4 Serron: I’ll be doing it with greater frequency from now on.

Star Trek is of course known for its technobabble - using technical sounding terms, often combining real words, to describe some aspect of the universe or more specifically a starship or some other piece of futuristic technology. The problem with technobabble is that too often it ends up being a term that technically savvy viewers can comprehend, and that actively comes across as unscientific nonsense.

"Hailing frequencies" is a term guilty of this. In current parlance, a communication frequency refers to the frequency of the radio (or other electromagnetic) wave that you are using to establish a communication channel. Both sender and receiver need to have their equipment tuned to the same frequency, or else the signal will be missed by the intended receiver.

You can achieve this in a few ways:

• The sender and receiver can have a prior agreement to use a specific frequency. This is like knowing the frequency of a radio station you want to listen to, and dialling it straight in.
• The sender can broadcast on a specific frequency, and the receiver can have the knowledge that they should sweep a range of frequencies to find the signal. This is like scanning with your radio to find a station that sounds good.
• The sender can broadcast over a wide band of frequencies, in the hope that the intended receiver will be listening in on at least one of them. This doesn't have a common analogue in the real world, because for the vast majority of applications the sender and receiver of radio communications on Earth have enough prior knowledge that one of the above methods will work.

The last method does have an application if you don't know anything about the intended receiver at all. A case might be if you are shipwrecked and have a radio capable of transmitting on many frequencies. You want to contact anyone who might be listening, but don't know what frequency they might be listening on. So you try them all. (This is assuming there isn't an established emergency distress frequency, which for many cases in modern transport there is.)

The other case might be if you're trying to contact an unknown alien species who you anticipate has radio receiver technology. This is a case which arises in Star Trek.

Now, the command "open hailing frequencies" could be the correct future jargon command for "send a wide-band signal to see if they are listening, and then if they reply we can narrow down to the correct frequency to use". But to our present ears it sounds more like "send a signal on frequencies that are designated for hailing unknown vessels". And unfortunately the idea of pre-designating specific frequencies for hailing unknown vessels is ridiculous, as there's no plausible expectation that any unknown aliens will use the same frequencies for that purpose.

So it sounds a bit off. They could easily have come up with some other jargon term that made more sense.