please tell what does negative sign indicates in the expression of faradays second law

The negative sign indicates that the induced emf will always OPPOSE the motion or change which started the induced emf in the first place. This is according to Lenz's Law which states that: "the direction of an induced emf is such that it will always opposes the change that is causing it".

So with a decreasing current the induced emf polarity will be acting as a source(to delay the decrease of current) and with an increasing current the induced emf polarity will be acting as a load(to delay the increase in current). In either way the induce emf opposes the change.