Check out example codes for "difference between signed apk and unsigned apk". It will help you in understanding the concepts better.
Code Example 1
Signing Android apps is just like signing any other application; it provides the user with some level of assurance that the code hasn't been tampered with since you released it. Unsigned apps should be usable generally but do require lowering the security level. A quick Google search found a number of articles that explained that all you have to do is check the Unknown Sources box in Settings->Security.
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