Some useful ways to use Synths 

When we run code like 

{SinOsc.ar}.play 

we make a Synth (which you see counted on the synthesizer graphic under Synths). We'll see soon enough ways to make reusable recipes for synthesis (SynthDefs) but we'll stick with the quick {}.play formulation for now. But let's just show a few things you can do here. 

We can store a reference to a running Synth to send messages to it later. 

a = {SinOsc.ar(440)*0.1}.play

a.run(false) //turn off synthesis (saving CPU) without freeing the Synth

a.run //restore (defaults to a.run(true)

a.free //stop it explicitly: note that I didn't need to use the 'stop all' cmd+. or alt+. key command, and can individual kill specific Synths this way

Arguments 

An argument to the function {} is turned by the SuperCollider language into a control, a way for you to change a parameter of your Synth later on. 

a = {arg freq=440; SinOsc.ar(freq)*0.1}.play

a.set(\freq,330) //change frequency! 

A common mistake is to try to pass a UGen as an argument: 

a.set(\freq,MouseX.kr(220,440)) //WON'T WORK, WILL CRASH 'ERROR: can't set a control to a UGen'

You must have plugged together all the UGens earlier on within the {}. More explanation of this later on (it's really to do with the language being separate to the synthesizer and the Synth mechanism having to be used in certain ways). 

Two arguments at once:

a = {arg freq=440, amp=0.1; SinOsc.ar(freq)*amp}.play

a.set(\freq,rrand(220,440), \amp, rrand(0.05,0.2)) //change frequency and amplitude randomly within a uniform range; run this line multiple times