Can someone explain the unit building cool down to me?

More or less. Bottom left corner of your in-game screen has a population counter. Shows a little guy running and some blocks. As you build units, this counter turns from green -> yellow -> red (this includes harvesters!) . A green counter is an almost instant rebuild. A yellow takes a few second, and red is basically not going to happen unless you wait a good solid minute.

Now the inverse of this is also true. As you LOSE units, the bare depletes and you can rebuild the units quicker.

What this does is prevent one player from spamming lower cost units and swarming, or getting a harvester kill and out spamming you with high cost units. You both have the same unit pop to work off of. (harvesters pending)

As the game progresses, this cap is lifted so more units can be rebuilt instantly. Around the first nuke you can usually work with about 4 units. 2nd nuke is 5ish and 3rd nuke is 6. This is why you will see good players hold pads with their harv so they have more combat units at the other pads, or they will sac or let their harv die on purpose because they think they have enough money to make it through the rest of the game.

So the trick is to learn when to hold on to almost dead units to hold pads, and when to throw them away to either build a fresh one, or to replace it with a different counter.

Then the game ONTOP of the game is learning when an enemy unit no longer poses a threat, so you ignore it and let it live so it eats up your opponents pop cap. Things like an almost dead riflemen shooting at your harvester. Just leave em be.

So in your example above, you had a double harvester, which means you are down a pop cap from your opponent, but you have higher income. The problem is this allows him to get more 2v1's on your units so he gets more value out of the fights, offsetting your income advantage. On a side note (if you made it this far) the income ALSO increases as the match goes on. So going straight for double harv isn't as advantageous at the start because it is still slow. But around the time the 1st nuke goes off, it sees a boost. double harv at this time is actually quite strong.

So double harv builds usually go 1 harv, delay the nuke as long as possible to get the income ramped up, if you lose 1st nuke the pressure is on, get the double out and make sure to delay for your tech to get out now. If you secure 1st nuke you can sit back leisurely, delay and get your tech out to stop them.