What exactly is an interface.
In this case an interface is a box or card that connects external gear to the computer and vice versa. You did not say which Mac you have but I assume in your setup it would be a box.
For music we deal with two types of data: audio and MIDI. We can only hear audio, MIDI data are used for technical communication between MIDI gear or between MIDI gear and software.
Every modern keyboard needs MIDI, because the computer must know which keys you are pressing down and how fast and how long you do that. In Logic you can play software instruments via MIDI. You play on the keys and the sound is made by Logic.
However, if you have a Synthesizer with a nice sound like your Triton, you may want to use its audio signal too. To get audio into the computer you need an audio interface. So technically you need two interfaces: an audio interface and a MIDI interface. One is to convert analog audio data to digital data and vice versa, the other is for transporting MIDI data to and from the computer. Most interface boxes have both built-in.
Your Triton has already a MIDI interface and it works over USB. This is what you use right now. But there is no audio going over this connection, only MIDI.
The Mac has a built-in audio interface. The most simple solution for you would be to buy an appropriate cable and connect the audio output of the synthesizer to this tiny audio input on the Mac. But we cannot recommend this method because if you have a Triton, you want to record it's good sound and the internal audio interface is not very good.
Furthermore, you want to hear your music in good quality. The Mac does not deliver a good sound on its own. This affects monitor boxes and headphones. And last but not least, one day you may want to record over a microphone. For that you need also an audio interface.
what audio interface do you recommend
There are so many available in all price ranges and with different features, it is hard to recommend a specific model.
I think a Presonus Firebox would be good for you. It is Firewire, does not cost a bargain and has a very good sound overall. I just found out that
Sweetwater sells it currently far below the normal street price, for just US$ 200. If you are in the States and this price is acceptable for you, the Firebox would fit all your needs today and tomorrow.
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If you do not plan to expand your setup in the future you can of course get any cheap interface with a reasonable sound. Just be aware that the audio interface will become the central hub of your setup. It is responsible for the quality of in- and outging sound. Good mics, good speakers, good sounding synths, nice headphones, nothing works well if the interface is bad.
To give you a starting point for working with an external audio interface, below are two drawings that are basically identical, the only difference is the MIDI dataflow. You have to decide yourself if MIDI over USB or MIDI over standard cables are better for you.