A chord can be described in a number of ways, and I will describe them in more and more detailed and mathematical ways as you progress through UkeSchool. Here in the beginner's lessons, we can think of a chord as 4 notes, one note for each string, all played at the same time. The first chord you are going to learn is called the "C" chord, also known as the "C Major" chord.
A chord diagram is a simple drawing of the instrument, with markers for the finger postions required to make the chord. At UkeSchool, you can click on any chord chart with your mouse, and it will play the chord for you! Try it! We also have colour in our chord diagrams - in this case, the 'gold' colour means the 'ring finger' - that shouldn't be too hard to remember. You'll learn about more fingers as you learn more chords.
To play the "C" chord as shown in this chord diagram, you simply press the end of your ring finger down on the "A" string between the second and third "fret", those metal bars that go across the neck. You have to have your finger far enough back from the third fret that the string can vibrate freely, and you have to press down hard enough that the string stays down on the fret.
Now, drag the fingernail on your right index finger down across all four strings - you've played your first chord! Does it sound the same as when you click on the chord diagram? If it doesn't, you probably need to tune again. Don't feel bad if you do - it'll take a little while to get everything settled. Take a deep breath, tune up each of the strings with the uke tuner, and try the chord again - once you get it sounding nice, you'll be ready for your first jam session!