What is mean by lead management and their functions?

If you are referring to "sales lead management" then there are actually 5 stages;

1. Lead Generation
2. Lead Qualification
3. Lead Distribution
4. Lead Verification
5. Lead Conversion

Lead generation is the front end process used to create a lead - can be a marketing led function or sales led.

Once a lead is generated you need to determine if it is real, the level of interest and what needs to happen to it.

You then need to distribute it or pass it onto member of the sales team who will also need to verify the lead is real and sales ready.

If the lead is sales ready it can then be placed into a sales process to convert that lead into a customer if there is a good fit.