“You should put some stuff on it to keep it from getting infected, don't you think?”

Your brother cut his finger. He just put a band-aid on it, but you think he should also use an ointment so that the cut doesn't get infected. You say:

You should put some stuff on it to keep it from getting infected, don't you think?

some stuff

"Stuff" is a general word that means something similar to "things". But "things" are countable, while "stuff" can also be used for liquids or uncountable objects.

In the example above, the speaker says "some stuff" because she doesn't know exactly what the kind of medicine that's used for cuts is called. The correct name for the "stuff" she is thinking about is "ointment", but this word is a little uncommon.

keep (something) from (doing something)

When you don't want something to happen, you try to "keep it from" happening:

You have to stir it to keep it from getting burnt.

I tried to keep him from leaving, but he was determined to go.

This phrase is used in spoken English. A more formal phrase with a similar meaning is "prevent (something)":

You should apply an antibacterial ointment to the cut to prevent infection.

get infected

When a part of your body "gets infected", it means that bacteria enters it and causes problems like pain, redness, and strange fluids.

You can also say that a person "gets infected" when they get a virus or other contagious illness.:

Over 3,000 people in New York have been infected with the bird flu.

(sentence), don't you think?

You can end your sentence with "don't you think?" in order to get someone to agree with your opinion. This is useful for when you're making suggestions, like in the example above.