
quatschen
to chat
Colloquial for to have a relaxed and extended conversation; to chat.
Wir haben gestern Abend stundenlang am Telefon gequatscht.
We chatted on the phone for hours last night.
to chat / talk nonsense
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to chat
Colloquial for to have a relaxed and extended conversation; to chat.
Wir haben gestern Abend stundenlang am Telefon gequatscht.
We chatted on the phone for hours last night.

to talk nonsense
Colloquial, often disparaging for talking nonsense or stupid stuff.
Hör auf so einen Unsinn zu quatschen und konzentrier dich!
Stop talking such nonsense and concentrate!
The verb 'quatschen' (to chat/talk nonsense) clearly belongs to colloquial language. It is used in everyday, informal conversations and should be avoided in formal contexts such as official emails or academic texts.
Lass uns später mal in Ruhe quatschen.
Let's have a proper chat later.
When 'quatschen' (to chat) is used in the sense of 'plaudern' (to chat), it often takes the preposition 'mit' (with + dative) for the conversation partner and 'über' (about + accusative) for the topic.
Sie hat am Telefon lange mit ihrer Mutter über den Urlaub gequatscht.
She chatted with her mother on the phone for a long time about the vacation.
The negative meaning 'Unsinn reden' (to talk nonsense) can often be recognized by the presence of a disparaging object such as 'Unsinn' (nonsense), 'Mist' (crap), or 'dummes Zeug' (stupid stuff). Sometimes it is also phrased as a command.
Er quatscht nur Blödsinn, wenn er aufgeregt ist.
He only talks nonsense when he's nervous.
to chat
Both words describe a relaxed, extended conversation. „Plaudern“ sounds friendly and neutral, whereas „quatschen“ is noticeably more colloquial and can also sound disparaging, depending on the tone. „Plaudern“ is a good choice for a friendly conversation, but „quatschen“ is also possible among friends.

Nach dem Essen quatschten wir noch entspannt über unseren Urlaub.
After dinner, we chatted casually about our vacation.
Nach dem Essen plauderten wir noch freundlich über unseren Urlaub.
After dinner, we chatted amiably about our vacation.