The chenille part of the letter is the actual yarn in the letter.
The trim or "chain stitch" is the actual stitch that borders the letter. 95% of letters will have the same color trim as the chenille, but in some cases, the trim can be a different color in order to show a contrast.
1st layer of felt
The first layer is the piece of scrim felt that is directly sewn to the actual chenille. When looking at a letter with two layers of felt, it would be considered the first or innermost border.
In many cases, this can be the same color as the chenille to give it a fuller look.
2nd layer of felt
The second layer of scrim felt is the bottom most piece of felt that would be considered the outermost border.