The full condition 1 is as follows:
Maintain a level of academic achievement consistent with your previous coursework in your fall and spring courses of your last year of school (as listed on your UC application) as preparation for success in college. A decline in the weighted term GPA by a full grade point may result in the cancellation of your admission.
The further explanation is:
We expect that the grades you will earn in your senior year will look similar to the grades you earned in the first three years of your high school career. For instance, if you were a straight-A student for three years, we would expect A’s in your senior year. Consistency in your level of achievement must be carried through your senior year coursework.
I read that as saying that if your cumulative weighted GPA for freshman through junior year (ie not your UC GPA) is say 4.0, you shouldn’t earn less than (or equal to) a 3.0 weighted GPA in either fall or spring term of senior year (remember that UC doesn’t see or consider fall grades during admission). It’s not the average weighted GPA for the whole of senior year. But including freshman year when most people have fewer weighted courses should hopefully give a bit more margin than a comparison with your UC GPA.
If you fell short of that then you should be prepared to file an explanation (and perhaps an appeal if the explanation isn’t accepted). Do you have a good reason? For example, a friend of my S successfully appealed the withdrawal of his UC offer (after he failed a class) by providing documentation of a concussion that affected his studies.