Pay for the first SIP is a convenience extended by the Mutual Fund companies to investors which enables you to pay for your first SIP instalment at the time of setting up the SIP. 

For example, on 20th July, you have set up a SIP for the 5th of every month starting 5th August 2020. You can pay for the 5th August 2020 SIP instalment today itself, ie 20th July. The next instalment due will be for 5th September 2020.