Assertiveness is about being clear, honest and confident in expressing how you feel or what you want in such a way that other people listen and understand with clarity and respect for your position.
Being assertive comes from knowing how you feel, knowing what you want or want to happen (the outcome from the communication) and being clear about what needs to happen to achieve that.
For some people the thought of being assertive makes them feel uncomfortable because they don't want to
offend or have their motives misunderstood. When you are assertive it is impossible to offend anyone because you are merely expressing your right to be heard, listened to and respected, in the same way that you would react to their own 'assertiveness'.