A Bitcoin wallet consists of two keys:
The private key is like the main password that allows you to manage your funds. You should never show it to anyone. The public key is like a bank card number. It is needed for someone to send you money, but on its own, it does not unlock anything.
The private key is simply a very long random number. A public key is created from it using complex mathematics, and then your Bitcoin address. But the process only works one way: it is impossible to determine the private key from the address.
Is it possible to guess the private key?
Imagine that the private key is a combination lock with 256 rotating dials, and each of them has 10 numbers. To open the lock, you need to guess all 256 numbers. The total number of combinations is so enormous that:
If every person on Earth (about 8 billion) had a supercomputer. Each of these computers would check billions of keys every second. And they would work together for as long as the universe has existed (13 billion years). They still wouldn't be able to check a tiny fraction of the possible combinations.
What if luck is on your side randomly?
The chance of randomly guessing a private key is about the same as:
Pointing to a specific atom among all the atoms in the universe.
Or win the lottery a billion times in a row.
It is impossible to randomly find someone's private key or guess it. The space of options is so vast that all the power of humanity would not be able to solve this task even over the entire existence of the universe. This is what makes Bitcoin secure.