lauter
louder
The comparative form of 'laut' (loud); describes a higher volume or intensity of a sound.
Kannst du bitte die Musik etwas lauter machen?
Can you please turn the music up a little louder?

louder / nothing but / honest
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louder
The comparative form of 'laut' (loud); describes a higher volume or intensity of a sound.
Kannst du bitte die Musik etwas lauter machen?
Can you please turn the music up a little louder?

The form of 'lauter' reveals its meaning. As the comparative of 'laut' (loud) or with the meaning 'honest', it is declined like a normal adjective. In the meaning 'ausschließlich' (exclusively) or 'nichts als' (nothing but), it is an invariable particle and is never declined.
Er erzählte lauter lustige Geschichten.
He told nothing but funny stories.
In this meaning, 'lauter' is the comparative form of 'laut' (loud). It is used for comparing sounds, often in the structure 'lauter als ...' (louder than ...).
Ein Düsenjet ist viel lauter als ein Auto.
A jet plane is much louder than a car.
As a particle, 'lauter' means 'nichts als' (nothing but) or 'ausschließlich' (exclusively) and emphasizes a large amount of the same thing. It stands directly before a noun (usually in the plural) and always remains unchanged.
Auf der Party waren lauter nette Leute.
There were nothing but nice people at the party.
The meaning 'ehrlich' (honest) or 'aufrichtig' (sincere) is rare and belongs to elevated written language. You often find it in literature or formal speeches to describe a person's character.
Sie war eine Frau von lauterem Herzen und reinen Absichten.
She was a woman of honest heart and pure intentions.
only
'nur' and 'lauter' can both express that nothing else is included. 'nur' is very general and works with people, things, amounts, or actions. 'lauter' is used mainly before a plural noun or before a group when you want to emphasize that everything consists of the same kind. It often sounds a bit more lively or colloquial. So you can say 'nur Fehler' (only mistakes) or 'lauter Fehler' (nothing but mistakes). However, 'nur' can be used much more broadly.

In der alten Kiste liegen lauter kaputte Kabel.
There are nothing but broken cables in the old box.
Heute Morgen trinkt die Chefin nur einen Kaffee.
This morning, the boss is drinking only one coffee.