The idea behind absorption is that when we work on something we stick to the same task for some time, we don't change focus every second. So for example let's say you are writing a novel in Word. You are using Word for 20 minutes, then jump to Google for 1 minute and back to Word for another 20 minutes. Chances are that jump to Google is related to your novel. In ManicTime you can make a list of these tools which can be used for any purpose (like Google in this case) and ManicTime will absorb them into surrounding activity.
Lets take search engines for example, you never know what you will use it for, maybe it is work related, maybe it is to check the weather, buy a new camera... Whenever you find something which can be used for any purpose, put it in the 'General' autotag.
Similarly you will have some tools, like Notepad or Windows explorer, which you know you use for work, but don't know in advance on which project. Put these in 'Work tools' autotag.

The key to absorption working reliably are rules which can only be one thing (let's call these rules 'hard rules'). For example these are rules like:
A visit to facebook.com -> autotag as 'Browsing'
Using outlook on support@manictime.com email -> autotag as 'ManicTime, Support'
Using Excel with documents in folder c:\accounting -> autotag as 'Accounting'
...

When generating autotags, ManicTime first calculates these, then it will turn the ones which are set for absorption into one of these. 

In the example below the red and green autotags are based on some rules, the blue highlighted one is Google search.

Now let's set 'General' autotag to be absorbed by all other autotags.

This is what it looks like after the absorption is applied.

So now Google search was transformed into one or the other autotag. This might be a strange analogy for a computer application, but autotags set for abosorption are similar to a chameleon, they will change color to whatever is in their vicinity.

Please note that while autotags can be correct, they can also be wrong. That visit to Google between working on a project could easily not have been related to a project, to which it was applied.