........... Hard cap is when you hit a specific # where more crit has zero value (or in practical terms, negligible value even if it is non-zero).

Soft cap is when you hit a specific # where more crit has a "lot less" value for some random reason.

This reason is due to various mechanics in the game system. In the simplest example, a dev may actually artificially code lower value after a certain point (various games have had mechanics where crit after an arbitrary number -- 40% for example -- had half value).

There are other examples -- in certain games where successful crits influenced the chance to hit, crit would have a higher value early on where you replaced "misses" with "crits", and at a higher value, you would only replace "hits" with "crits". This later value is less meaningfully less than the initial value. This instance of a crit softcap is much less "artificial", but still happens.

There are some examples of statistical mechanics which will cause a sudden drop or a partial drop in crit value. For example, if you have an ability that procs when you crit, but has a hard cap of 1 proc per 5 seconds, then crit chance has a "softcap" when you reach the threshold of 1 statistical proc per 5 seconds. Prior to that level, crit will increase your chance to crit AND increase your chance to proc. After that level, crit will only increase your chance to crit -- your chance to proc that effect isn't increased much. This sudden change in crit value is a softcap.