This compression system is a very handy invention, especially for Web users, because it lets you reduce the overall number of bits and bytes in a file so it can be transmitted faster over slower Internet connections, or take up less space on a disk.
Concrete hardens to 600PSI (Minimum) after 24 hours. This means that 5 three grown men could all stand on a pogo stick (if that were possible) who's base covers 1 square inch without harming the concrete. You walking with shoes on exert about 10 PSI, well below anything that could damage the new pour.
It depends on the composition of the concrete mix, water content, and temperature. Generally, concrete sets in 24 to 48 hours allowing for you to walk on it and partially cured within a week (keep the cement free and clear of heavy equipment during this period) at which you can continue building/construction. Most mixes fully cures at 28 days.