An adjunct or adjunct argument is a non-core role in a sentence. This means that it’s just an extra bit of information that isn’t required for the sentence to make sense. The opposite of this is a core argument, which is required for it to make sense.
In the sentence ‘I saw a dog in the park on Sunday’, ‘a dog’ is not an adjunct. If you removed it, you would get ‘I saw in the park on Sunday’, which makes no sense. But ‘in the park’ and ‘on Sunday’ are adjuncts. If you removed them, the sentence would still make sense.