LLMs are so-called Large Language Models. In Q1 2024 they are the chic.
LangChain
https://python.langchain.com/docs/get_started/introduction
LangChain describes itself visually as a 🦜 and a ⛓️ . A parrot and a chain. That’s almost poetic 
LangChain is a framework for developing applications powered by language models. It enables applications that:
Are context-aware: connect a language model to sources of context (prompt instructions, few shot examples, content to ground its response in, etc.)
Reason: rely on a language model to reason (about how to answer based on provided context, what actions to take, etc.)
Keep in mind: the LLM
The use cases vary, but in the following it’s about creating
a GPT-4 prompt answering machine (you may also use other models, such as GPT-3-turbo etc. Incl. models from other vendors)
a GPT-4 Python code and documentation generator (you may also use other languages, such as Java etc.)
a GPT-4 Python agent which tests and executes code
other agents can compare documents, draft Emails in MS365 / Gmail etc.