Loaded "hot" means that the ammo has more than a standard charge and the result typically is a greater amount of velocity (usually the intended benefit) along with being louder and having a sharper recoil (negatives). This is all fine so long as the ammo stays within the design parameters of the gun. Too hot and if you exceed appropriate chamber pressures, you risk blowing up your barrel and gun with a catastrophic failure. Even ammo that is within parameters can be problematic as it puts additional wear and tear on a gun above what normal ammo would do, sort of like driving your car really hard versus normal driving.
The term "hot" is often used as a qualitative assessment of an ammo without any knowledge of the ammo's load or velocity. You will read people posting that such and such ammo is 'hot.' Often what is being stated is that the person heard it was hot, but has not chrono'd it himself. Where seeming verification comes in is when the person shoots the ammo and notes that based on his shooting, it is 'hot.' At that point, the person is basically suggesting that since the particular ammo had more recoil than expected on some level, that the ammo must have a greater than normal load and by default must have a higher than normal velocity.
What a lot of folks fail to understand is that what feels "hot" is not necessarily "hot" or may not be 'hotter' than some other brand of ammo that does not feel as 'hot.' For example, if you do a search on the words 'blazer' and 'hot' here at 1911forum, you will find a lot of folks who consider Blazer .45 acp ammo to be 'hot.' Using the original standard of 850 fps, Blazer does run a little 'hot' sometimes, somewhere between 840 and 890 fps in a full-sized 1911 depending on who chrono'd it. This happens to be about the same range for a LOT of .45 acp ammo. So in relative terms, Blazer isn't terribly hot compared to standard loads.
The "feels hot" assessment is one that CANNOT be counted on as an accurate assessment of ammo velocity or of powder load. Often one ammo that feels 'hotter' than another brand actually isn't any faster. How is this possible? Easy, it is due to powder burn rate. Slower burn rates will give the impression of a round being loaded light (cooler, not hotter) when this may not be the case at all. Some of the 'hotter' ammo has a snappier recoil, but actually doesn't push out the ammo any faster. The ammo simply gets up to speed faster is all, but is not going any faster after leaving the barrel.
As a final thought, consider differences in velocities for 'hot' ammo over normal ammo (assuming normal is 850 fps). Say the ammo chronos at 875 fps, such as you might get with Blazer ammo in a full-sized 1911. The increase in velocity is LESS THAN 3%. That is a very small amount to actually be feeling in the recoil of the gun in order to assess velocity. Feeling 'hot' isn't a valid ammo performance assessment. You have to chrono. With that said, felt recoil is a very real attribute to consider for gun/ammo handling characteristics, but not bullet performance.