A major chord is a triad.
A triad is a chord with three notes.
In a major chord the three notes are the root, the major third, and the perfect fifth.
A root note is just the bottom note of the chord.
It's the note that the chord is named for.
A major third is four half steps above the root.
The major third is what gives a major chord it's 'happy' sound.
The perfect fifth doesn't really change the way the chord functions, but it reinforces the strength of the root note.
Here's a major chord spelling cheat sheet.
Note that: # means sharp, x means double sharp, and b means flat.
These are all in root position.
C major: C E G C# major: C# E# G# Db major: Db F Ab D major: D F# A D# major: D# F# A# Eb major: Eb G Bb E major: E G# B F major: F A C F# major: F# A# C# Gb major: Gb Bb Db G major: G B D G# major: G# B# D# Ab major: Ab C Eb A major: A C# E A# major: A# Cx E# Bb major: Bb D F B major: B D# F# If you want to really get good at your major chords, practice spelling them.
Write out a list of roots and spell the rest of the chord.
It's a bit boring and repetitive, but in the long run it pays off.
Learning how to spell major chords is one of the cornerstones of understanding music theory.