Manual or copy/paste✔

Pick this option if you want to manually type in the 81 character seed. But remember that it is actually "hard" to type in something truly random with a qwerty-keyboard, as most people statistically favor certain characters based on their location on the keyboard, and that humans tend to avoid certain combinations such as recurring characters or characters in sequence (eg. ZZZ and ABC respectively).

This is also the option to select if you have already generated a seed somewhere else, like for example 'Keepass', and want to copy/paste it in here.

Crypto + user entropy ✔

With this option, you will use javascript's crypto.getRandomValues() in addition to user added time entropy.
Each tryte of the seed will randomly switch up to 100 times/sec, and upon user interaction (entropy as random time by user), a tryte is picked.

This option should introduce entropy at a level just as high or higher than for example random mouse movement.

There is also an option to autogenerate, where the user interaction is replaced by randomizing the interaction time between 130ms and 970ms. Don't use it if you don't trust the code behind it.