A breath controller is a specialised device that you can use to control Midi parameters whilst recording in order to mimic, very convincingly, the articulations of wind instruments.
It consists of flexible tubing with a mouthpiece on one end and a USB gizmo on the other.
A breath controller is something you don't have to learn to play, it's very intuitive. But, you do have to find the most expressive parameter to control and that depends on the VSTi you are using.
Cubase has some native VSTi's with articulations that can be controlled easily and seen directly in real time on automation lanes (their instrument name in Media Bay is suffixed with VX). You just record using the read/write automation just as you would with the midi track. Also look at quick controls at the bottom of the inspector tab by default.
There is a bit of setting up needed to use CC midi channels but my DAW found it out of the box (you need to install drivers and an app first). Plenty of videos on BoobTube to explain better. Some VSTi's like Kontakt have a 'learn' facility built in to their instruments to assign the controllers to parameters. You simply right click a rotary button (or a fader, or other control), blow into the BC and, like magic, it controls the button.
You can use the BC to add expression as you play in real time or go back to already recorded tracks and add expression retrospectively. Oh no, you'll be going back and remixing all your old tracks!
I do think it can be so subtle that you might not get any benefits with horn sections. But for solo instruments it really works.
The one I use is the BC is 99 Euros www.tecontrol.se/products I have no connection whatsoever with this company so I am fairly objective, all in all, it has good documentation and I can recommend it.
Of course you don't need a breath controller, you can hook up your mod wheel to midi CC's to get the same effect without spending money but it is intuitive and produces a smooth curve.
For live work it's a must if your horn player doesn't turn up, I saw Eric Clapton's keys guy play a horn solo that was indistinguishable from the real thing using one!