How can manage methods in background?

Question

For example we have a lot of tables in database and when program(mainform) get open, a lot of data from tables adding to list or arraylist and ... .
How we can manage this problem to avoiding program bad opening?

Answer 1

Using threads?

Answer 2

yes i heard about it, but dont know how work exactly? can you tell me what happend when we use threads??

Answer 3

Instead that explain (its very unpractical in a comment) i link an online resource https://docs.oracle.com/javase/tutorial/essential/concurrency/