the general sentiment

The "sentiment" of a conversation or discussion is the general feeling of the people who are talking. A "sentiment" can be positive, negative, or neutral.

The word "general" often comes before "sentiment". The "general sentiment" is how most of the people in a group feel about something.

it's hard for me to (do something)

When a lot of people can't do something, say "it's hard to ___":

It's hard to find a job in this economy.

But if you have trouble with something on your own, say "hard for me to":

It's hard for me to understand what they're saying when they talk so fast.

concisely summarize (something)

To "summarize" means to explain something in a short form. For example, you can summarize the plot of a movie or what was said in a meeting.

The word "concise" means "short" when you're talking about writing or explaining things. It's a positive word, so being "concise" in writing or speaking is good.

"Concisely summarizing" something means that you explain it in a very short form.

what they're saying

The word "what" turns this expression into something like a noun.

Some people make mistakes like:

summarize they're saying

But you can't do this. You need to include "what" or "that which" or "the thing that".