Originally Posted by: johnv31552
I must be dense as I look at this subject. I understand that there is a 1 4 5 for every key, and I know those chords.[/quote]
Do you know why those specific chords are the I-IV-V of the key?

Originally Posted by: johnv31552I still don't understand what it means to play in a specific key??
[/quote]
To be "in a key" means that you are using the notes of a specific scale as your standard. It means that most or all of the notes of that scale are used in the piece of music you are playing.

To be in the key of C major means you are using the notes of the C major scale as your standard. You will use notes of that scale to form the melody of the song, notes from that scale to form the chords of the song. If you deviate from that key you will be modulating from that scale & then returning to that scale as the "home key".

Alternately, if you are in the key of A major, then you are using the notes of the A major scale as your standard. You will use notes of that scale to form the melody of the song, notes from that scale to form the chords of the song.
[QUOTE=johnv31552]Let's take the song House of the Rising Sun for an example. Based on the chords used which are Am C D F and E, I assume that this song is played in the key of C.

"House of the Rising Sun" is in the key of A minor. It uses the notes of the A minor scale as it's basis. All of the notes of the melody are from the minor scale. Most of the notes of the chords are also. There are modulations: D major has an F#, E major has a G#. But those are temporary deviations that give character to the song.
[QUOTE=johnv31552]
Assuming someone says let's play it in the Key of A, how do those chords change?

You could use a capo. That makes it easy to use the same shapes. But more fundamentally you have to know the interval distance of the key change.

House of the Rising Sun is in the key of A minor. If we want to play it in B minor, then we measure the interval distance from A to B. Two half steps. We move all the chords up two half steps.

Amin goes to B min
C goes to D
D goes to E
F goes to G
E goes to F#

If we want to play it in C minor, then we measure from A to C. One and a half steps.

Amin goes to C min
C goes to E-flat
D goes to F
F goes to A-flat
E goes to G

If we want to play it in D minor, then we measure from A to D. Two and a half steps (or an interval of a fourth).

Amin goes to D min
C goes to F
D goes to G
F goes to B-flat
E goes to A

The concept that unites all these chords is that the House of the Rising Sun uses this chord progression:

i - III - IV - VI - V

So you pick a root note, then measure the distance from that root note to figure out which chords to use for that specific key.

These tutorials cover the basics of music theory & the circle of fifths. They explain all these concepts in detail.

https://www.guitartricks.com/tutorial.php?input=495
https://www.guitartricks.com/tutorial.php?input=835