It stands for Point in Time. It provides a basis of comparison, regardless of the period you select. So, if you were to select a monthly comparison, the PIT numbers would reflect the status at this point of time in the month, compared to the same point in time in the previous month.
This is a useful metric as it allows a user to assess the trend of a website where certain metrics are concerned. For example, were you to do a comparison of KPIs based on a quarterly period and see that the previous quarter marked a drop in traffic over the quarter before that, then the PIT numbers would allow you to quickly assess whether the current quarter was likely to follow suit or whether your site's visits were 'bouncing back'.
You can see that illustrated in the above example. This site's organic visits (sessions) experienced a downswing of -13% in Q4 2017 when compared to Q3 2017. Now, if you hover over the 31.5k figure above those numbers, you can clearly see that this represents the traffic for a specific period - in this case, from the 1st October, 2017 to 12th November, 2017, so roughly 6 weeks into Q4. Now, if you then hover over the 36.4k figure, you can clearly see that the site has received 36,368 organic visits over a similar period in the current quarter (roughly six weeks into Q1 2018 - 1st Jan to 12th Feb). This is therefore a sign that the site is 'bouncing back' after a dip in Q4 2017.