what is IRR?why is it often judged?

The internal rate of return (IRR) is a capital budgeting metric used by firms to decide whether they should make investments. It is an indicator of the efficiency of an investment (as opposed to NPV, which indicates value or magnitude).
The IRR is the annualized effective compounded return rate which can be earned on the invested capital, i.e. the yield on the investment.
A project is a good investment proposition if its IRR is greater than the rate of return that could be earned by alternative investments (investing in other projects, buying bonds, even putting the money in a bank account). Thus, the IRR should be compared to an alternative cost of capital including an appropriate risk premium.
Mathematically the IRR is defined as any discount rate that results in a net present value of zero of a series of cash flows.
In general, if the IRR is greater than the project's cost of capital, or hurdle rate, the project will add value for the company.
Surveys indicate that executives prefer IRR over NPV. Apparently, managers find it easier to compare investments of different sizes in terms of percentage rates of return than by dollars of NPV. However, NPV remains the "more accurate" reflection of value to the business. IRR, as a measure of investment efficiency may give better insights in capital constrained situations. However, when comparing mutually exclusive projects, NPV is the appropriate measure.
In addition if the NPV of one project is higher than another and the other project has a higher IRR, then the cross over point method can be used to solve this dispute.
Cross Over Point > IRR = Accept project with higher NPV and if the Cross Over Point < IRR = Accept project with higher IRR