what is this plug for

[kirscht]

 

[toms89]

Looks to me like you circled a protective boot on the hot terminal from the battery. (Mines missing so not sure) The solenoid it is located on is considered the starter motor relay.

 

[Jay87LX]

Yes, Its not a plug, Its just a rubber boot over the hot side of the starter solonoid. Many people loose them