There is a subtle difference in meaning.

Please, help me to understand this

Means, literally, ‘please get involved in my process of understanding (the ‘learning activity’ involved in being able to understand’ - this).

It means ‘assist me in my journey of becoming someone who already understands’ or ‘is able to understand’ whatever it is).

Please, help me understand this.

Focuses more on the topic that is desired to be understood. Whatever ‘this’ is, in the discussion.

It means ‘please help me understand the topic - ‘cooking pancakes’ or ‘bevelling marble’ or ‘arranging tulips’ etc.

Examples ‘to understand’:

• ‘Professor - can you please help me to understand this trinomial equation method?’ (a process) ‘Yes, you must add a, b, then c’.
• to bus conductor ‘can you please help me to understand this?’ - pointing at bus map - process of how to use it. ‘What are the yellow lines? And what do the green lines represent?’ (process) ‘Yellow lines are day buses, green are night’
• ‘Can you help me to understand how I get to haberdashery via art supplies and fashion, in this department store, please? (‘Help me with my process’) ‘Up to 1st floor Haberdashery, right through Art, take the list to 3rd for fashion’.

So, the answer will ‘guide your journey’ or process. In becoming a person who is able ‘to understand’.

Examples ‘understand this’:

• Can you help me understand this? (Pointing at lumpy pancake batter) ‘well you put your milk in too soon. Should have mixed the eggs first. That’s why this is so lumpy!’ (Focus is on ‘this’ - the batter)

• Can you help me understand this? (Pointing at rough edge of marble bevel) ‘well you’ve got your chisel at the wrong angle! That’s why this is all rough!’ (Focus is on ‘this’ - the marble)

• Can you help me understand this? (Pointing at droopy tulips) ‘Well, you didn’t pierce the ends with a pin before putting them in the vase, did you? That’s why this wilting has happened!’ Focus is on ‘this’ - the tulips)

https://medium.com/@english_grammar/how-to-use-preposition-in-english-grammar-on-at-in-of-for-2fdb11e80029] Look at the definition of ‘to’.

There is a subtle difference in meaning.

Please, help me to understand this

Means, literally, ‘please get involved in my process of understanding (the ‘learning activity’ involved in being able to understand’ - this).

It means ‘assist me in my journey of becoming someone who already understands’ or ‘is able to understand’ whatever it is).

I am looking to acquire ‘a state’ of being able ‘to understand’.

Please, help me understand this.

Focuses more on the topic that is desired to be understood. Whatever ‘this’ is, in the discussion.

It means ‘please help me understand the topic - ‘cooking pancakes’ or ‘bevelling marble’ or ‘arranging tulips’ etc.

Examples ‘to understand’:

• ‘Professor - can you please help me to understand this trinomial equation method?’ (a process) ‘Yes, you must add a, b, then c’.
• to bus conductor ‘can you please help me to understand this?’ - pointing at bus map - process of how to use it. ‘What are the yellow lines? And what do the green lines represent?’ (process) ‘Yellow lines are day buses, green are night’
• ‘Can you help me to understand how I get to haberdashery via art supplies and fashion, in this department store, please? (‘Help me with my process’) ‘Up to 1st floor Haberdashery, right through Art, take the lift to 3rd for fashion’.

So, the answer will ‘guide your journey’ or process. In becoming a person who is able ‘to understand’.

Examples ‘understand this’:

• Can you help me understand this? (Pointing at lumpy pancake batter) ‘well you put your milk in too soon. Should have mixed the eggs first. That’s why this is so lumpy!’ (Focus is on ‘this’ - the batter)

• Can you help me understand this? (Pointing at rough edge of marble bevel) ‘well you’ve got your chisel at the wrong angle! That’s why this is all rough!’ (Focus is on ‘this’ - the marble)

• Can you help me understand this? (Pointing at droopy tulips) ‘Well, you didn’t pierce the ends with a pin before putting them in the vase, did you? That’s why this wilting has happened!’ Focus is on ‘this’ - the tulips)

https://medium.com/@english_grammar/how-to-use-preposition-in-english-grammar-on-at-in-of-for-2fdb11e80029] Look at the definition of ‘to’. While ‘to understand’ is an infinitive, the word ‘to’ is still in there, with its meaning of ‘heading towards’ something.