what does it mean when somebody says it's gone pear shaped?

It means something went wrong or didn’t go to plan. In British English, “it’s gone pear shaped” is an informal way to say a situation has failed, turned messy, or ended badly.

Example

• “We were supposed to finish the project today, but it’s gone pear shaped.”

Tone

People often use it in a slightly light, conversational way, not usually for very serious situations.

In plain English

• Good: things are working.
• Gone pear shaped: things have gone off track or collapsed.

TL;DR: It means things have gone wrong.