What a summary is for
A good summary answers one question: what is this about, and do I need to read the rest? It gives the main event, the people involved and the outcome. It is a map of a piece of writing and not a replacement for it.
What usually survives
The core facts survive a summary best: who did what, when and where, and the main result. Figures and names that carry the story tend to stay, because they are the most important and the easiest to select.
What usually gets lost
Nuance is the first thing to go. Caveats, disagreement between experts, uncertainty and background that explains why something happened are all longer than a sentence, so they are cut. A summary can therefore sound more certain than the original really was.
How extractive summaries work
Some tools write a new text. Others select the most important sentences from the original, which is called extractive summarising. The advantage is that every sentence in the result was written by the original author, so nothing is invented. The trade-off is that the sentences may feel slightly disconnected when placed together.
How to use one safely
Use a summary to triage: decide what to read in full, catch up quickly, or recall a story you already know. Before relying on one for a decision about money, health or law, read the original. A summary is a starting point and sometimes missing the one detail that matters.
Writing your own
The best test of whether you understood an article is to summarise it yourself in two sentences. If you can say who, what and why it matters without looking, you have got it. If you cannot, that is a sign to go back to the original.