Explanations:
1♦(4+ cards)
1♠ (5+ cards - almost certainly due to the note below exactly 5 and not of great quality)
3N in the unopposed auction 1♦-1♠-3N would show a big 4441 with diamonds, spades and one of the others, S thought this still applied, N didn't so S alerted and bid 4♠
The other thing you need to know is that NS play somewhat unconventional weak 2s, so S as dealer at game all would have opened any 6 card spade suit, and any good 5 card spade suit unsuitable for a rule of 19 1 opener (and not containing 2 aces).
Is pass a LA here or is 5♦ automatic ?