What is meant by escape from all signs? Good will , compassion , equanimity etc are objects of meditation. 
Good will is used to escape from ill-will. Compassion is used to escape from viciousness. Equanimity is used to escape from passion. 
All the above ideas are familiar to me and I understand them well.

Along the similar lines there is a concept of signless (I guess signless is also an object of meditation). Signless is used to escape from all signs. 

Following quote from aṅguttara nikāya explains signless :


  “Furthermore, there is the case where a monk might say, ‘Although the
  signless has been developed, pursued, handed the reins and taken as a
  basis, given a grounding, steadied, consolidated, and well-undertaken
  by me as my awareness-release, still my consciousness follows the
  drift of signs.’ He should be told, ‘Don’t say that. You shouldn’t
  speak in that way. Don’t misrepresent the Blessed One, for it’s not
  right to misrepresent the Blessed One, and the Blessed One wouldn’t
  say that. It’s impossible, there is no way that—when the signless has
  been developed, pursued, handed the reins and taken as a basis, given
  a grounding, steadied, consolidated, and well-undertaken as an
  awareness-release—consciousness would follow the drift of signs. That
  possibility doesn’t exist, for this is the escape from all signs: the
  signless as an awareness-release.’


I am having difficulty in understanding what are signs?

My question is : what is signless (or what is sign) and what is meant by escape from all signs?
According to Mahayana interpretation, "signs" or "marks" are concepts, or conceptual designations, also known as preconceptions.

The problem with conceptual designations is that we tend to confuse them with reality (=to reify; noun "reification"). To give a canonical example, when someone is attached to his own personal stereotype of what an "attractive woman" should look like, when he meets one that matches his stereotype he no longer sees her as a complete person, instead he only sees his own concept of woman, he sees a sexual object! Then we tend to look with our conceptual brain, instead of looking with an open mind. Then, all we see is our preconceptions or biases - instead of seeing what actually is. Not only we see nothing but our preconceptions, we completely confuse them for reality, we think that what we "think" is what really "is" - but it is not, we see our own mental fabrication! Then on the basis of that fabrication, we develop certain attitude toward the object - for example we want to make acquaintance with the woman, with an implicit intent to get close with her. This is called "following the drift of signs".

Another example of this is racial stereotyping. You see a person of a national origin that you generally associate with ignorant violence - and then all you see is your preconception, you no longer see the real person. On the basis of this you develop aversion to this person, which ends up causing a conflict.

This happens with inanimate objects as well. Once we know that a given object is "a tree" - we no longer really see it as-is, we see our concept: "a tree". It takes a special effort to unsee our abstract concept and see the actual tree.

According to Mahayana, the entire Samsara is an illusory world completely made of such concepts! The notion of "I" or atman is one of such preconceptions. It is on the basis of attachment to preconceptions that we have unsatisfied craving, which manifests as suffering. Complete cessation of reification is "escape from all signs". 

This is a rather simplified presentation, it actually goes much deeper than that, but this can get you started. Entire Mahayana was born from study and development of this teaching, passed in the oral form from Buddha's students to next generations. In Mahayana we refer to this as "shunyata" and "prajna-paramita", but it is also present in the Pali Canon in quite a few places like this one, using terms like "groundless", "having no position", "signless" etc.