If the forms are understood, two people can understand each other. If they each believe in something else or their own brand of justice, real communication cannot be achieved.
The forms to Plato were the most basic and everlasting things we could know. The idea of forms is vital to Plato's idea of immortal soul. In the Phaedo Socrates and Simmias discuss the immortality of the soul at great length. Since the forms have always existed and will always exist, so must the soul. On page 126 this is clarified:.
Then the soul is immortal?.
Yes, He said.
And may we say that this is proven?.
Yes, abundantly proven, Socrates, he replied.
At this point, a transition can be made to the theory of recollection. Since the soul is immortal and has always existed, Plato believes that when we are born (something that has happened more than once according to Plato) we possess all the knowledge we will we ever need. However, in the process of being born we forget most of this knowledge and spend most of our life recovering what we once knew. Plato also says that a lot of what we already know can be retrieved through inquiry. For example, we are capable of recalling justice when we contemplate "what is justice?" A passage on page 90 helps clarify Plato's idea of knowledge and birth.
And if we acquired this knowledge before we were born, and were born having it, then we also knew before we were born and at the instant of birth not only equal or the greater or the less, but all other ideas; for we are not speaking only of equality absolute, but of beauty, good, justice, holiness, and all which stamp with the name of essence in the dialectical process, when we ask and answer questions. Of all this we may certainly affirm that we acquired the knowledge before birth.
This passage reiterates the previously mentioned general idea of recollection. It also mentions essence, which is just another term Plato uses for form.
