Sfr1224 is correct. Head angle is an important part of frame geometry that you're more likely to feel when you ride. It's only one aspect of frame design, but it's a biggie.
Slack head angles do more than slow the steering response, though.....they also have the effect of slightly shifting the rider's weight rearward, further behind the front axle. This, combined with the typically longer fork travel that accompanies slack frame geometry, helps with rider balance and impact absorption when your bike is pointed downhill. Another effect it has is to lengthen the wheelbase, which provides some stability at speed and in cornering.
The steeper the terrain that the bike is meant to handle, the more slack the head angle. You can feel two degrees of difference if you know what you're looking for. Serious downhill bikes may have heads as slack as 64°. Light freeride and 5"-6" trailbikes will hover around 66°-69°, and xc/race frames will stick to the more traditional 70°-71°. Road bikes, in comparison, are usually 71°-74°.
Steep head angles are really responsive, and there are some dirt jump and trials bikes with a steep head that are really nice to maneuver. They make riding through tight, twisty trails a little easier, but if you're in rough terrain or steep terrain, they make it a tad easier to wash out the front wheel in a corner or to go over the bars when you get surprised by obstacles (however, slack head angles won't cure endo-sickness....fair warning. LOL) :o)
In contrast, if you put a slack head angle bike on flat ground, the steering will feel sorta dead, and if you turn the bars hard, there's a "breakover" point where the wheel meets resistance and then snaps past it....kinda sucks until you get used to it. As an example, a couple years ago I bought a freeride hardtail frame....very burly, and I wanted to set it up for urban drops, etc. I was used to xc frames with 70°ish heads, and this was a 67° bike designed around a 130mm fork (5"). It felt bad to me, and wasn't what I wanted for riding the ledges on square planter boxes, quick maneuvers on stairs and tight cornering.....or for trackstands, pivots, etc. So, I put a 100mm fork on the bike which had the effect of steepening the head angle by about 1°. Changed a couple other aspects on the stem/handlebars, too, but it improved the handling for my purposes. You don't want to vary a whole lot from what the manufacturer designed (and putting longer forks on bikes designed for shorter ones is generally a bad idea for safety/durability).....but there's a little room to play.
There is a lot more to frame geometry and what-affects-what, how the bike feels and handles, etc. Chainstay length, seat tube angle, and bottom bracket drop (height) are all important numbers to pay attention to if you want a particular type of frame/handling. It gets a little complex, but manufacturers have tweaked geometry into different variations that work very nicely for their applications. It's funny, though......in a couple hundred years or whatever, the initial design of the bike frame hasn't changed very much at all. Whoever came up with it was either lucky or pretty intelligent! :o)