Chord symbols are often a part of sheet music and usually appear above each line (music staff), above each individual measure. There can be as many as five or six of these for the same measure.
For the most part, they are there as a guide for “playing along” with the music in the sense of accompanying the written music whether sung, played on piano, or both. These symbols are essentially designed for a guitarist, but can be played by other instruments such as a ukulele, banjo, mandolin, etc. Sometimes they include the finger positions for a guitarist (to play the proper chord).
Because they are only symbols, it’s up to the musician to play each chord in whatever way they choose. For example, the chords could be played by strumming or finger picking a guitar, playing the chords in Bluegrass style on a banjo, or playing the chords in a very rhythmic fashion on a mandolin.
This, of course, all depends on the accompanying musician knowing how to form and play each chord (symbol). Some times, there are chord symbols the musician is not familiar with (and does not know how to play). Using the example of a guitar player, they will have to look up the chord (symbol) in a chord book, or create the chord using the sheet music as a guide.
An experienced musician will play the chords in a way that compliments the (piano/sheet) music. Their playing should match the tempo of the music and compliment it in style and tone. This requires being familiar with a particular piece of music and how it is intended to be played. One of the challenges of playing along using chord symbols is avoiding “conflict” with the sheet music. Care must be taken to play the proper notes of the chord and avoid disharmonic “clashes” with particular notes of the music. For example, playing the note B when the melody asks for the note C, or playing an E minor chord when the melody asks for an F#.
Using a guitar player as an example, chord symbols allow the guitarist to sing along with their own accompaniment. The guitarist may know the piece of music and how it is sung, or if they can read music, find and sing the melody as it appears in the music. Professional musicians often use chord symbols to perform a piece of music they are not totally familiar with. For example, if a vocalist can sing the melody as presented in a piece of the sheet music, the guitarist needs to know only the chords represented by the chord symbols.
As examples, Fm7 means an F minor 7th chord, Bb6 means a B flat sixth chord, Adim means an A diminished seventh chord, and G means a G major chord. Having chord symbols can also help the piano player. Knowing the chord name as it is played on the piano is a great learning aid. But care has to be taken as the chord symbols are usually a very simple version of the chord on the piano.