in most of the tricep excercises it says that u r not supposed to lock ur elbows when u go up..y?

bad for joints is what they think

It's bad for the joints and the ligaments. Go really close to lockout, but not all the way.

in most of the tricep excercises it says that u r not supposed to lock ur elbows when u go up..y?

Well after you have worked out for a few months 4-6 months at the very least you will begin to know what specific excercises work for you. What excercises give you that tight feeling and get the blood in the muscles.

With that said I've noticed that locking out in any excercise releases tension and makes it easier to do (taking longer to get a good workout in). In dips if you totally lockout you will do more but they will be easier. Try and do them without completely locking out, also change the pace from time to time to shock your muscles i.e. sometimes do your reps slowly and sometimes explode.