So, to sumemrize,
P-Locks is the name for the fact that you can basically change every parameter on every step. This is easily the most powerful function of elektron sequencers.
You select the step and change all the parameters you want and then the step will play like that. You can do this for any step.
Trig conditions are a special parameter (also per step) that allows you to change whether a step is played or not.
You can set it for instance to 50% and then there will be a 50% chance that that step will play when the sequence comes around.
You can also set things like ‘play every 3rd of four times the sequence is run through’. Then the sequencer will ignore the step on the first two times the sequence plays, then play it on the third time, then ignore it for one more sequence loop, then start over again, ignoring the first 2, play in the 3rd runthrough, ignore once more, etc.
That way you can make for instance a 16 step sequence play something different after a few cycles.