Are these related "performSelectorInBackground" and "performSelectorOnMainThread"?How does it exactly works? [duplicate]

This question already has an answer here:

• Whats the difference between performSelectorOnMainThread and dispatch_async on main queue? 2 answers

Its in login process

if(![[_emailText text] isEqualToString:@""] &&![[_passwordText text] isEqualToString:@""]){
        [self performSelectorInBackground:@selector(loginInbackground) withObject:nil];
        // Hide Keyboards
        [_emailText resignFirstResponder];
        [_passwordText resignFirstResponder];

        // Show the logging in screen
        [_loggingInView setHidden:false];
    }

They have used performSelectorInBackground,and after this

-(void)loginInbackground{
    // Add the username and password to the keychain
    keychain = [[KeychainItemWrapper alloc] initWithIdentifier:@"login" accessGroup:nil];
    [keychain setObject:@"loginData" forKey:(__bridge id)kSecAttrService];
    [keychain setObject:[[_emailText text] lowercaseString] forKey:(__bridge id)kSecAttrAccount];
    [keychain setObject:[_passwordText text] forKey:(__bridge id)kSecValueData];
    keychain = nil;

    if([self loginwithuser:[_emailText text] andPassword:[_passwordText text]]){
        [self performSelectorOnMainThread:@selector(loginCompletewithbool:) withObject:[NSNumber numberWithInt:1] waitUntilDone:TRUE] ;
    }else{
        [self performSelectorOnMainThread:@selector(loginCompletewithbool:) withObject:[NSNumber numberWithInt:0] waitUntilDone:TRUE] ;
    }

What does performSelectorOnMainThread do?