The exact number is 1 ounce to 28.3495 grams, but unless you're very good at maths that isn't too helpful. For most cooking purposes, I find that if you think of 1 ounce as about 25 grams, that's quite accurate enough and that makes it much easer to calculate. So when you look at a packet of meat, say, 100 grams is about 4 ounces or a quarter pound, 200-250 grams is about 8 oz or a half pound (227 is the more accurate figure) and so 450 grams is close enough to one pound.

Once you remember this you can make your shopping decisions more easily, and if you do need to be more precise - for example, with some cakes I would want to be really exact - a quick look at an online conversion chart can be very helpful. But I always keep a paper one in the kitchen cupboard too, just in case.

The ounce is a weight or volume unit which is used in several different systems. Below is the explanation from Britannica:
Ounce has been used for measuring weight or volume in many countries. Though the ounce is no longer a legal unit in the UK since 2000, it is still commonly used in the US for measuring weight. 1 ounce is about 28 grams.