Can someone explain to me what deltas are, what they do and what they are used for. I´ve been trying to understand it long time, and seen many tutorials where deltas are being used for varios purposes But I still cannot fully understand it.
Event tick for example has a delta output. I´ve seen many other functions and nodes with delta inputs or outputs. Are all these referring to the same “delta” or is there different ones?
Is it important to know about deltas, when making blueprints?
It’s a word mathematicians invented in place of the word “difference” to feel better about themselves 
Delta time literally means time current frame took to compute, or time between the finish of last frame and finish of the current frame. Delta time means “difference” from last time. In the similar manner, when you see something like movement delta, or rotation delta in context of some transform function which is being called each tick, it just means difference from the previous frame position to the current frame position.
So simply translate delta to “difference from” 
Of course it doesn’t always apply to frame times. For example Delta (Rotator) function in BP will take 2 rotators and return their delta, meaning the difference in the angles between them.
It makes a little bit more sense now. Thanks.